Yeah, I agree with SM. The Aliens in Rez were still hard to kill. That one scene where one got shot in the head and then exploded, I think anything being shot at that range would've reacted the same way.
One thing I don't like is the Marine's ability to "kick" an Alien off of him or like in AvP where a Marine could "Block" an incoming Alien assault. Not realistic if we go by the film canon.
Now, if Gearbox add a feature that allows an Alien to knock your gun out of your hand and you have to scramble to regain it or cycle through for your backup to save yourself, that would be a neat feature that has yet to be done with these games. Cause, the Alien is all about surprise, so I think less Aliens would be better, and have those few Aliens literally come out of nowhere and make your life hell. The reality is that you could not just walk backwards shooting at an Alien as it jumps around in front of you and just takes it. You want them to snatch you up out from the walls, the floors, the ceilings and if they are coming at you from the front, they lunge at you with claws extended and teath bearing.
THAT is what Colonial Marines should all be about. The horror which made ALIEN and ALIENS so damn scary.