As an almost-exclusively Alien player on the PC versions of AvP and AvP2, I beg to differ: Aliens' mobility has always been underrated, and they actually are very powerful. Only in certain situations, such as against an extremely good Marine sniper on an open map, or against a pistol-spamming Predator on a low-player-count enclosed map, are Aliens handicapped.
Granted, the Aliens in AvP were unbelievably fast, and in AvP2 I almost always played as a Runner. Speed is life!
(Incidentally, my favorite MWLL mech thus far is the Osiris B. :)