Off the top of my head.
*The campaigns are too short.
*The plot is a little over the top as you have bond villian type mad scientists.
*Aliens and Predator that can be close to lava (In real life you will die just by being within meters of the stuff, it is that hot)
*The Alien playstyle is horrible, they are supposed to be 8ft tall monsters yet you are forced crawl on all fours making you the shortest.
*There is gameplay glitches in multiplayer and the multiplayer itself is broken, you have to wait for the next match rather be able to just jump in one.
*The combat is glitched as well, the distance tracker is non-existent as you can hit and be hit by someone that is meters away from you. I do mean in melee of course.
*Then there is a few issues with continuity on the timeline and then there is the fact that it follows the AVP movies.
It is a fun game, it is just far from flawless. It is better than ACM at least.