Broken Base: Gamers that bought Assault Horizon BEFORE picking up any of the other games will love it. Most long time fans hate it.
The biggest break point is the DFM system. Not only in this game, but arguably in the entire franchise. Supporters love it because it makes dogfights very spectacular and intense, plus it encourages to use it to people who in previous games just used missiles over and over since using the machine gun was way too hard for getting the same results. Detractors hate it because they think it destroys any sense of skill or challenge, and turns dogfights into glorified on-rails shooting segments, if not down straight Quick Time Events. The fact that some climatic boss fights forces you to use it doesn't help.
Stopping an ICBM launch by slipping under the missile's wake and accelerating upward after it, getting close enough to take damage from the thing's exhaust.
Engaging in a dogfight within the outer edges of a Category 5 hurricane.
Rescuing Guts when his ejection system is damaged by using his cannon to shoot off the canopy on Guts' Raptor.
D-Ray gets one himself, when, during the Moscow assault, enemy long-range SAMs take out several helicopters and force all other choppers to duck down to avoid being targeted. Realizing that the mobile SAM teams will obliterate the fighters when they get back from refueling, D-Ray and his gunner Jack hunt down and destroy all the mobile SAM teams. And then, just as a final kicker, devastate the remaining rebel forces, essentially winning the battle single-handedly.
Complacent Gaming Syndrome: Expect Capital Conquest to be the game mode that's frequently played online, mostly due to the discovery of exploits that allow completion of the mode within minutes.
Game-Breaker: Those Quick Air-to-Air missiles that were dreaded online and were the most broken missiles in the series? Yeah, they've been buffed back to AC04 status, and when in Dogfight Mode, the reticule stays glued to the enemy plane you're targeting.
The penultimate mission, "Hurricane," fictionally takes place on January 2016, around the East Coast of the United States, where a hurricane designated as Alex brewed up and partly hampered your gameplay. Fast forward to the real life January 2016, Hurricane Alex brewed up in the North Atlantic part of America, admittedly a rarity among hurricanes.
Memetic Mutation: In the form of an inter-fandom Borrowed Catchphrase, expect the common reaction to "White Devil" to be "IT'S A GUNDAM!"explanation White Devil, the title of the music playing during Mission 09: Siege, is the also official nickname of Amuro Ray due to his exploits in the RX-78-2 during the One Year War, though it's more popularly attributed (in fanon) to Nanoha Takamachi, who happens to be heavily inspired by the Gundam franchise to the point that some fans refer to her as a human Mobile Suit. Humorously enough, the site of the mission is a Russian airfield known as Belyi Base. The word belyi, or bélyj, is Russian for "white," thus giving you... White Base, just like in MSG. Ace Ventura references are a distant second.
Mis-blamed: As stated in the trivia section, Jim DeFelice had plans for a story set in the Strangereal setting, but the decision to go into the real world was Project Aces'. Forum hopping on GameFAQs has people up in arms with regards to DeFelice's story for the game.
Considering the Strangereal nations are still based on modern or historical real-life countries, the concerns may or may not have remained legitimate had this game gone with the Strangereal setting. Besides, most of those criticisms lie anyway with the facts that the War Is Hell aspect of the series is much toned-down, the storyline spends too little time outside of Bishop's personal conflict (whereas those who already played any or all titles between 04 and 6 would likely expect much more than that), and that the player character is Bishop himself and therefore no longer stays anonymous in Assault Horizon.
Also, while there is significant vitriol towards Dogfight Mode mechanic, what is the the recommended method of ensuring the best kill probability in the PS2 trilogy? Getting right up behind an enemy plane, in gun range. Which is exactly the same as DFM, only manual.
They don't automatically home your plane in on your target like DFM do, mind you.
DFM was not the problem. The negative feedback was virtually nonexistent when the trailers were announced. It was after the fact that DFM was actually also well-disguised cutscene/Quick Time Action and mandatory in places to boot, then it became a problem. To compare, every older game, has you go up against an Ace squadron (or even a final boss a la Pixy) and ask you to shoot them down with your own skill. DFM is a tool, but the final boss fight against Markov is a forced on-rails segment that has to be done with DFM. If it were that DFM stayed as a tool instead of a glorified cutscene, then there will be much, much less negative reaction to it. After all, this is an expanded version of the one seen in Sky Crawlers: Innocent Aces and that was not received poorly.
Narm: The aforementioned Fist Pump by Bishop at the end of the story, a minor meme in its own right.
Similarly, even Joint Assault wasn't the first game in the real world. The arcade games (Air Combat and Air Combat 22) took place in the real world, but they're so obscure only a few people even know those games exist.
Polished Port: The PC port adds a bonus content pack (extra planes, maps, skins and skills upgrades) and support for screen resolutions up to 1900x1200.
Scrappy Mechanic: The Close Range Assault mechanics are rather contentious, especially Dogfight Mode.
Tainted by the Preview: The demo forcing you to use Close-Range Assault to splash aces has not been warmly received.
It doesn't help that you have a whopping five scripted cinematic sequences before his plane is actually vulnerable. DFM is already contentious with fans because of its nature as a disguised quick time event. Have fun needing to do five of them before you're allowed to kill Markov.
Of course, this fight can be cheesed if you either a) bring an aircraft that has ECM, or b) learn to exit Dogfight Mode whenever you hear the lock-on beep and enter it again to make the missiles useless. Or you can do both to be sure.
That One Level: The AC-130 Level is damn near impossible in Ace Difficulty, because there's no way to prevent being shot at with SAMs from the ground and the SAMs are too fast to be shot. Couple that with the fact that you have to deal with two or three at a time, that two kill you and that you have to escort your allies, it's pretty hard.
The helicopter levels are often criticized for dragging on far too long and ending up dull and repetitive as a result. The fact that helicopter special weapons (of which you are only allowed to carry one of, despite the helicopter having multiple hardpoints and the ability to carry AT missiles and rocket pods is standard on all attack helicopters) have a small ammo pool and will run out means that you'll be forced to use the chin-mounted gun halfway through the mission, with no resupply option.
They Changed It, Now It Sucks: Where to start... Many AC fans are unhappy about the change in setting, the lack of superweapons, the introduction of Close-Range Assault, the Regenerating Health and the addition of piloting different kinds of aircraft like helicopters and bombers, for starters.
Another large point of contention is Bishop not being a Featureless Protagonist, a break from (nearly) every other ace you play as in Ace Combat. Even worse, not only was Lt. Col. Bishopa rather bland and generic military hero, but Warwolf 3 and Warwolf 4 are pretty much an afterthought throughout the entire game; they're never seen or named, unlike Guts, and don't get any substantial dialogue until the final two missions of the campaign. With only a few tweaks to the story could either of them proved perfect for the spotlight had not Bishop been the viewpoint character. The immersion players felt in previous games was sorely lacking and dearly missed. It's telling that while Infinity is also set in our world, the Player Character is a return to form (and also a Mobius1analogue), with the Bone Arrows' flight leader Viper serving the authoritative role Bishop was arguably better suited for.
H.A.W.X. was deemed inferior to Ace Combat as a franchise by AC fans, yet many old-time AC fans think that HAWX is superior to Assault Horizon. It's more of a "It's a completely different game with a completely different gameplay" than complaining about the changes, though.