I think the problem is the alien team returns a whole lot more for increasing levels of skill in the player, as well as giving the complete newb a hefty edge... although a complete newb won't stand a chance against a 1337 (see definition below) group of humans. If you pit 1337 humans vs 1337 aliens, IDK... humans would most likely win up till S3 if they camped, then it would be aliens all the way (because of the tyrant)
The dretch is hard to hit in the first place. 1337 player = lethal weapon. The Basi is a bit easier, but in the right hands, when used in a firefight, it wreaks havoc. The Mara in the right hands can bypass base defenses and take out the RC in a couple of runs if the defenders are otherwise occupied (say by those nasty goons). The average / above average Goons can easily breeze through 10 straight kills unless the hummy players have above average amounts of skill. Rants don't need explanation.. other than its interesting that they use the tactics that should be used by lower classes of aliens to make base attacks, thus assuring they never die unless something upsets their rhythm.
The Rifle is a powerful weapon for 1337 players... IDK how they do it, but almost every shot seems to hit, no matter what. @ HP/shot, that's lethal. The shotty is, of course, devastating, and 1337 players seem to be able to do dodge attacks pre-emptively (whereas I find that even though my dodge, according to my screen, should have saved my hide, it doesn't... in case you're wondering, I do it all the time in Halo to avoid 'Hogs, but in Trem, the aliens must have uber-oversized slash / chomp ranges.. which would be fine if their animation or w/e showed on-screen their damage range) Mass Driver is a good sniper weapon, but 1337 players use it like a single-shot Rifle, never missing. Pulse Rifle is well-balanced, but somehow some players seem to be able to shoot anything that moves with it, which is normally impossible because of its slow pulse travel rate... it's just weird being shot out of the air with a PR as a dretch. Chainguns are offset by their inaccuracy, but somehow some players can nail anyone within seconds... IDK how, I've only been able to do this a few times by accident. (With chainsuit, ofc, everyone knows that normal soldiers have to crouch to control the kickback.) Flamers are flamers, they seem pretty fair. Lucys can be like the pulse rifle... some players can land their shot whereever you are going to be by the time the shot arrives, even though you're doing your best to have a random attack pattern. :sigh: IDK
Of course, any group that has a chain of command and / or good communication (TeamSpeak or similar) set up is going to have a huge advantage.
Definition of 1337 as I use it here: by this I mean you can't tell they aren't using an aimbot unless you spec them, they can chaingun an average tyrant player to death 1 vs 1, they can mass driver a dretch doing random jumps off of a wall mid-jump, and a mara doesn't stand a chance against them (unless it's another 1337 player controlling the mara), and with a lucy / flamer / chaingun and a couple of buddies they are absolutely unstoppable. As a goon, they are nearly unstoppable, as a rant, they are (they slash almost before you come into their field of view, killing you... they always seem to hit you where it hurts, taking all but the BSuit down with one slash - BSuits take two or three, IIRC).
If none of this makes sense, well, I'm still trying to figure it out myself.
But I know that average aliens vs average humans = aliens win hands-down. Newbs vs Newbs, not sure, depends on teamwork, I think. 1337 vs 1337... like I said, depends on the stage.