Aliens have too much advantage
Why? Because of their low health? Or because they don't have any ranged weapons?
Aliens have three "special features":
- they're fast
- they can walk everywhere (including ceilings)
- they can jump (about 10 m high - that's 33 feet for the non-metric world^^)
That's it. But they have two major disadvantages! 1. They're exclusively melee fighters. Spot an Alien from far away and it's toast. 2. Health is very low... One hit with the disc, shoulder cannon (charged), spear gun, plasma pistol, grenade launcher, Sadar, Skeeter or (not sure about the last one) a grenade from the pulse rifle and you're dead. Minigun and flamethrower will bring down an Alien in almost no time. A charged hit from another Alien's tail will kill you too.
If you learn how to play as an Alien, it's fun. A charged tail attack from behind is an instakill for Aliens and Marines and brings down a Predator's health a lot. But it is a melee attack... And thanks to the Alien's low health (and the huge variety of weapons that could one-shot an Alien) you can't do a frontal attack! The Alien needs its speed and ability to walk on walls. Otherwise it would be useless.