Honestly, I think that the game is dead even. If aliens use teamwork, marines HAVE to be tight to win, and they have to be good. Spec LM sometime. If the aliens are playing as a team, getting people on target on time, getting a hive and multiple fades at the 4 minute mark, the marines will always have one hell of a fight, and more often than not, will make one slip up and fail.
I played a game today where the marines ninjaed a phase into Surface Access, got a HA train inside the hive. I umbraed the hive, called for backup. Within 30 seconds, the whole team was inside SA, killed the phase first, the killed each HA off one by one. As skulks, a lerk and a fade.
I've changed my mind. IMO: Shotguns are the only thing keeping NS balanced.