The trouble with an early human rush (on ATCS) is that they have to run all the way back to base to refill/regen/upgrade gear, and the farthest forward they can reasonably get is right outside of the alien base, where they end up having to camp.
Aliens don't have that issue, since they can just pull back a moment to regen, then jump back into the fray, and it's a simple matter to get by the initial 3 turrets and munch on spawning humans, hop out of the base for a second, go basi/mara and end it.
Personally, my favorite thing to do as aliens is dretch flank through the middle the second I spawn, and attack all the humans camping the long hallway... easy mara or goon in the first 10 seconds of the game.
Edit in response to the following post:
The problem with humans storming into the alien base right away (on atcs) is that acid tubes slow them down, and do at least 4 dmg. This means that only 1 shot is required for a defending dretch to kill a human, while it would take the human 5 shots to take down the dretch. Also, dretches have the advantage of radar, and close quarters inside their own base. So unless you're looking to give the alien team 2 goons with no positive return for yourself, an early human rush (inside of the alien base) is foolish, and dare I say it? Not pr0.