These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Awesome Music: Way too many instances to count. Tetsukazu Nakanishi (primarily in the first 4 games) and Keiki Kobayashi (primarily from 5 onwards) are some of the best composers in the industry, and they lead teams of equally talented musicians. Every level has a unique background track that will make audiophiles wet themselves. Please list examples on the Crowner Page.
Broken Base: It took all of 10 minutes for the fan community to start tearing itself apart when the teaser for Assault Horizon was released.
Forget Assault Horizon, do you like the real world setting or Strangereal? Better yet, which game is the series' Magnum Opus?
Disappointing Last Level: The US and EU release of Electrosphere suffers from this and Ending Fatigue (for what little storyline there is... try wrapping your head around that one). The last 7 or so levels are still the same quality and design as the rest of the game, but each level keeps ending with "To Be Continued", leading to the next level without a chance to switch planes or even save, and the player begins to wonder when it will finally end.
Do Not Do This Cool Thing: The series insists, firmly and often, that War Is Hell. However, you play as an Ace Pilot, arguably the most glamorous combat role of all time, and frequent radio chatter indicates that your arrival on the battlefield singlehandedly restores friendly morale and causes enemies to panic. Also, as a Featureless Protagonist, you are spared the pathos-inducing family deaths that plague so many other characters. War is hell... for everyone else.
Goddamned Bats: Normal mook planes and SAMs when fighting aces at the same time. The ballistic missiles in Ace Combat 04 are just really annoying to shoot down. If you watch the flight path replay at the debriefing, you'll notice they goes side to side and up and down in a sawtooth pattern. Even the Game Breaker QAAM missiles have a hard time with this one.
The drones (and later UAV's) in 6 deserve mention for being ridiculously difficult to get a good missile launch on unless you're in a high maneuverability jet like the Su-33. The UAV's are worse in that they're only there to make the fight with Pasternak seem more hectic.
HSQ: Rises dramatically once you see the second trailer for Assault Horizon. Just watch.
Jump Scare: In the three PS1 games at least, the explosion of your plane when you crash or get shot down is EXTREMELY loud and can happen any second that you muck up, so it wouldn't be too surprising if it crapped you up at least once, especially since crashing in particular is usually deadly.
Amongst other examples is the one combining The Idolmaster and Ace Combat Zero.
Player Punch: The games love reminding you that the enemy aces were people with friends and families before you killed them, particularly the death in AC4 of Yellow 4, who goes down in one shot because her plane was badly maintained.
While the punch in 4 comes more directly, the hardest goes to Zero, no contest: While each game usually features one hidden ace in each mission plus a few boss aces, Zero contains 169 named enemy aces, and each and every one leaves you with their unlocked biography after you shoot them down. While a good many of them are merely shot down rather than killed in action, seeing names, ages, and life stories attached to each and every YY/ZZ/1995 - OPERATION X: KILLED IN ACTION is devastating, especially given that hardly a single one of them could be framed as a villain in any way other than that they took off from a different air field that day. Well, aside from Schwarze squadron, the Escapee Killers.
In Zero, Mission 12 is bad enough as it is. If you played 5 though and paid attention to dates, you saw it coming. What you don't see coming is when the wingman you had been flying with for every mission up to this point, Solo Wing Pixy, takes a shot at you and then deserts. That's one of the most personal betrayals in the entire series.
The 8492 squadron after Fortress in 5. If you do decide to take them on then be prepared for undoubtedly the hardest part of the game, period.
The Arkibrd. It launches UAV drones that constantly spam missiles and you have to destroy targets on the large ship that are underneath it. Except one of the targets is located on the front of the plane where its fuselage gives it a great deal of cover. Oh, and you're under a strict time limit, you have to basically destroy most of the Arkbird before it leaves the map (and it starts close to half way across).
Ofnir from the same game, primarily since you're forced to stay in the canyon, while they can go as high as they want.
Ilya Pasternak. While he thankfully doesn't use the Nosferatu's ADMM all that much, dozens of super-agile UA Vs will spawn with him, and both he and them can avoid missiles simply by accelerating. He's vulnerable only when he slows down to provide ESM for his drones, but within 5 or so seconds after that he'll jam your radar and fly so fast as to instantly vanish from the screen.
Sulejmani from Joint Assault. For most of the battle he has a rear mounted machine gun forcing you to use the machine gun and once it's destroyed, you're still better off using the gun because he moves so erratically that most of your missiles would miss anyway. All the while you're trying to avoid his missiles, which sometimes are being launched at point blank range.
Any level in the later parts of any of the games that required the player to fly into enclosed areas or shot them down if they flew above/below a certain height were usually reviled by players and often reviewers.
Let's not forget the U2 level from 04. An utter crapshoot with a time limit.
The one where you have to escort two cargo planes at different altitudes was worse. For some reason, the pilot of the plane that hasn't been damaged won't climb down a few thousand feet, forcing you into a vertical climb when you have to defend that plane after defending the other one.
"Four Horsemen" from 5: timing your attacks can be a bit tricky since Edge is counting down to target destruction, not firing, and missiles require time to fly. Also, this may be the only time in the series when it's important to fly at a specific speed (600 knots, give or take a bit). The PS2 has analog controls so you can control the throttle, but players might not notice that feature.
You think "Four Horsemen" is bad? Play Ace Combat X2's "Grand Flight". Let's see, you're flying an airliner in a canyon loaded to the teeth with defenses, and you have no escort whatsoever or any means of fighting back. If you make it through that, then you have to survive an ambush. This one mission alone is worse than every other example in the Ace Combat series combined.
Forget "Grand Flight", there's a mission in Ace Combat X that is even worse: TIME LIMIT. The mission in question is similar to Ace Combat 5's "Reprisal" in the sense you must stop the use of chemical weapons with a neutralizing agent. However, there's a nasty twist in this mission: go too fast or turn too hard, and you'll explode the agent. You basically have to watch yourself with the speed and angles while being hounded by enemy fighters, which is considered to be the most frustrating mission in the game if not the entire series.
They Changed It, Now It Sucks: Many hardcore AC fans were declaring that the series is dead upon finding out that Assault Horizon and Joint Assault take place in the real world instead of Strangereal.
A lot of fans also complained about the new Close Range Assault gameplay mechanic, despite the developers' claim that the "classic" AC gameplay is still intact and the new features are optional. To be fair, they weren't actually optional - even mooks would just drop chaff and throw off your missiles if you dared try to fight them normally. Infinity basically sold itself on the fact that Close Range Assault wasn't coming back.