First, a quick disclaimer. I'm not at all trying to brag, but this board seems to discredit your opinion if you have a low post count.
I've played for 6 months, choose auto-select everytime, equally skilled with humans and aliens. I was in STK, won our clan match against SGA, get top scores repeatedly. Look for me on Beer Garden, Yaknet or SST if you doubt any of this. So please, please, PLEASE don't tell me stupid things such as "Aim" or "Dodge"
Next, to sum up my view on balance: Skill is easier to achieve than teamwork.
As several of you have said in this thread alone, aliens take more skill. Humans are your standard FPS. Couldn't agree more...however, with that greater skill comes a much deadlier package - a stand-alone package.
Aliens can be deadly anywhere on the map, by themselves. Humans can be deadly close to their base if in great numbers.
I could extrapalate...s1 goons own anything the humans have, their s1 advantage gives s2 humans too small a time-frame to win the game before s3 comes rushing back in with alien superiority. SD? Well, the aliens have two bases, one surrounding the human base and one not-needed base back where the eggs are. Rarely are those luci's anywhere close to a hive or an egg. Human's can't get through waves of aliens with enough ammo to do any damage, anyway. But you've all heard that before. Recently, by me. I have a couple threads in the general discussion forum.
But besides that, just ask yourself the question - Which is in more abundance? Skilled players, or players that can work together and function as a team?