There is an old saying that goes like this, "engineer is only as good as your team" which I think is very true.
Engineer is ultimately a support class. A highly potent one if played properly but a support class nonetheless. You are at the mercy of the general flow of the game, if your team is getting rolled you will get rolled, if your team is crushing you will make the crushing more acute. If it's neutral, you will either help your team maintain the stalemate or if you are skilled you can give them a slight advantage. But I don't think in most cases that a single engineer ever really can affect the game enough to actually change the tide of the game in a meaningful way.
There's a few exceptions to this, certain maps are heavily dependent on having a teleporter (looking at you, blue side of deadwood until you get to last), and just maintaining a teleporter along can be the difference between winning or losing a game. But even then, I wouldn't say this changes the game significantly, you are just patching up a disadvantage your team had so they can fight on equal ground, not giving them an advantage.
I really enjoy playing engineer most of the time. To me, seeing the number go up on the teleporter, or seeing 5 teammates huddled around a dispenser gives me a feeling of accomplishment, and makes me feel like I'm contributing a lot to the game and that it's the reason why my team was able to win. That's at a base level of course, there's lots of other little joys that come with engineer, like fighting and winning against a class that would normally kill you, getting behind enemy lines with a sneaky teleporter, halting a push or singlehandedly defending a room with a well placed sentry, stopping an uber in it's tracks with a wrangler rescue ranger combo, and honestly just the little bit of dopamine that comes from setting up a complete nest in a way that I like, having figured out the positioning of my buildings and knowing the theory of why they should go where they go. Team Fortress 2 is just a well designed game that makes the regular tasks you do as a class satisfying and I like that engineer is a lot more strategy focused than just raw skill based, where you can gain an edge with map and class knowledge rather than just raw hours played. There's also a lot of small quirks about engineer that knowing about can give you a slight edge on, like teleporter ramp blocking, sentry hitbox masking, proper sentry rotation, etc.
That being said, to be perfectly honest with you a lot of the time playing engineer is pure suffering. I don't think this changes as you get better as engineer, and can even be worse as you get better (players who know you are good will try harder against you which makes your job that much harder). It's an unfortunate reality that engineer only gets worse over time as the playerbase learns how to play against him, to the point where now a days most players can roll through sentry nests without a second thought. It takes an order of magnitude more time to set up a sentry nest than it takes most players to destroy said nest, a problem made more acute by certain weapons (ie the Air Strike, Loch n Load, Direct Hit) but not by that much. Gone are the days where a sentry is much of a threat, it is really just best used as a tool for backline area denial. As mentioned previously, an engineer is only as good as his team, and this applies to the sentry too. If you have a good team that is covering various angles, and is attacking enemies who might be getting into good spam angles, a sentry becomes much more oppressive. But without your team helping to hold the angles the sentry is weak against, a single enemy player can obliterate your entire nest with ease. This often leads to frustrating scenarios as engineer where you are getting completely demolished by one guy while your team ignores you, and then they complain about the lack of a sentry/dispenser/teleporter.
I think there's a misconception that engineer is able to hold on his own. I do not think this is the case. I think that if you get even two enemy players with even accidental coordination they can take down an engineer of virtually any skill level if he's alone. It's simply not possible to deal with multiple threats if you are by yourself. But people tend to think that the engineer can single handedly hold off a push as if that's his speciality when it's simply not the case. Engineer is at his strongest when he has a team around him. The sentry is a very high DPS building but it's effectively made of paper to anyone who plays the game regularly. So a sentry is most effective when it is in a position where it cannot be easily harassed, such as when your team is standing in front of it, then it can perform area denial with little risk. Conversely, it does not matter at all how good of an engineer you are, if you are alone and the enemy team is pushing you, your nest will die and you have to accept that.
This problem tends to be more of an issue when playing against enemies who are specialists at making you not want to play the game. The various spy mains who frequent our servers, bombing beggars soldiers, good demomen, and the vast quantity of heavy medic combos will just eat you for breakfast. In many cases it seems almost unfair playing against these people (in particular beggars soldiers) but again I think it mostly just comes down to teamwork on the part of your team. While still possible, it is much harder for enemies to easily harass your buildings if your team is sitting in between them and you. Only spies will really bother going past your frontline to get to you, and a healthy number of teammates crowded around your dispenser will serve as a good deterrent for spies.
The best advice I can give as engineer is to always play around your team. If you are not near your team, you need to be. If your team is behind you, you need to run away because you're about to get your ass handed to you. If your team is really far forward, you should probably move forward too. Always play around your team, because you being there makes them stronger, and them being there makes you stronger. Neither of you benefit by you being far behind the lines.
And yes you will still get games that are demoralizing because your team does not play around your buildings and a single spy is taking you for a ride the entire match, or a single soldier just bombs you repeatedly and you can't even get a single building to level 3, but such is life. As with many things in life it is best taken cautiously and with the assumption that everyone around you is stupid and won't act in the way you want or expect, so despite it being seemingly obvious that playing around your engineer is powerful, most people aren't going to do that, nor should you expect them to.
When it comes to weapons, I think this is still an example of everything I've said above. The rescue ranger and wrangler will not make you able to solo hold against a team, you are still going to die. It only helps strengthen situations where your team is already covering your sentry, and helps you prevent your sentry from getting sniped or caught off guard in moments where your team moves away briefly. Personally, I'm somewhat indifferent to the wrangler and rescue ranger, I only ever use it when I feel like I have to ""tryhard"" and that my buildings are going to crumble if I don't, which often happens when my team is not playing around me very well. Otherwise, I'd much prefer to use the pomson and a pistol generally, because I prefer a proactive approach to engineer and taking out threats before they even reach my sentry. The rescue ranger and wrangler are not without their downsides of course, the wrangler you could argue is an upgrade to the pistol and I don't really disagree, it mostly comes down to preference but a wrangler and damage primary will still work out just fine. On the other hand, the rescue ranger is deeply flawed in that it is entirely a building babysitter gun, you cannot defend yourself for shit and any half decent spy player who sees you using the rescue ranger will take the opportunity to harass you hard, knowing that you aren't going to be able to defend yourself very well. This is mainly why I don't like using it and why I mostly just use it begrudgingly when I know I'm not going to be allowed to run around and gun people and that I'll have to babysit my buildings the whole round.
The last and probably most important tip for engineer is that you are more important than your buildings. It requires a bit of gamesense to know, but if you think your nest is about to get its shit pushed in, just run. Valiantly dying to get an extra wrench swing in on the sentry that was about to die anyways does not help your team at all. You are far more useful alive than you are dead, you can always rebuild your shit but the time it takes to respawn and walk back to the front lines is far greater. You can sometimes save a building with you as you run, such as bringing your teleporter exit or dispenser with you so you don't need to rebuild. But having your shit destroyed and needing to rebuild is part of playing engineer, and on most asymmetric modes like attack/defense or payload, the map is designed for you to eventually get pushed back to the last point where the actual winner is decided. Figure out how to retreat gracefully. If you see an uber coming in, and you know you're not going to be able to save your sentry, just run away as the uber is distracted with killing your sentry. If you see your entire team is dead and you are the only one left next to the point, that is a great time to run away with your dispenser or teleporter to set up somewhere else. One of the biggest causes for rolls on gamemodes like payload and attack defense is the defending team either falling back too far or not maintaining intermediate holds. Easy example is Badwater, the second point is a really good defensive position for red team, but if red is not ready and set up on the roof when blue comes, then they get rolled instantly. Same for the third point, it can be a potent defense spot if a red engineer can get there before blue does, but usually it doesn't happen because engineers on red tend to go down with the ship on the second point. Never go down with the ship unless it's overtime and you are about to lose.
For anyone else who reads this rant and is not an engineer player, here are some tips that will make your life better and the life of your engineer player better, they will be thanking you inside even if they don't say it
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Play around your engineer. If you are in a good spot, and your engineer is in a mediocre spot, I would argue it's better for you to also be in that mediocre spot (as long as it's not a literal death trap). If you leave your engineer alone, he will die, and without his buildings you will be weaker too. The two of you together are more than the sum of your parts.
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Help protect your engineer's nest. As I mentioned above, contrary to what you might think an engineer nest is pretty fragile and will fall in seconds if it's not being taken care of. Having a nest is very useful for you to recover health and ammo and as protection if you're another support class. If you see an airstrike soldier trying to rape your engineer, shoot him. If you see a scout repeatedly corner peeking and blasting a sentry, don't just walk by it. If you see your engineer repeatedly dying and losing his shit, help upgrade his buildings at spawn every once and a while. If you see a spy raping your engineer repeatedly, try spy checking more (this is a general tip).
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Help your engineer set up. Your engineer is most vulnerable when his buildings aren't ready, and some maps its hard to get ready before the doors open. Take 10 seconds to upgrade the teleporter entrance during setup time instead of sitting around doing nothing and staring at walls, or if they're doing a more advanced rollout, help them upgrade their other buildings. If you get forward spawn, thats another opportunity to upgrade a teleporter. But most important of all I would say is to defend your engineer when they are setting up mid round. For example, if you are about to lose a point and your engineer has fallen back to set up on the next point, go stand by him and prevent anyone from stopping him before he's done. Engineers in the middle of setting up their nest are one of the moments spies love to abuse the most, especially since often when a point is capped the spy is already moving to the next point to get the drop on a lone engineer. Don't leave your engineer alone to set up, if you leave him with his level 1 sentry he is going to get demolished by a single soldier, demo, spy, or heavy, and then you'll be sitting there wondering why you don't have a teleporter or a dispenser on the point and why you lost it so easily.