Assault Horizon was a spinoff that was released in 2011 between the mainline games Ace Combat 6 (2007) and Ace Combat 7 (2019). There are a few things that set it apart from the "mainline" games. Unlike most of the main games which take place in its own version of Earth called "Strangereal", Assault Horizon takes place in our world. You can also use aircraft other than fighter jets such as helicopters and the AC-130 gunship.
I think there a couple of reasons that this game isn't as highly regarded as other games in the series. Firstly, this game came out during the era that Call of Duty: Modern Warfare and Battlefield 3/4 were making modern military shooters very popular. Assault Horizon seems to take obvious influence from those titles for its story and tone. Secondly, this game introduced a system where dogfights would go into a cinematic "Dogfight Mode" where you were essentially on rails while the camera zoomed in to right behind your craft. This system was mandatory and turned off players who preferred the game play of the mainline games. At the time of this game's release, it was unknown if Bandi Namco was going to continue the series in this direction or not.
Unfortunately I couldn't find a better quality video despite scouring YouTube. I try to use the official uploads whenever possible but this game isn't on the Project Aces channel.