I think the balance is not that bad. Many things could be improve, such as the blaster that is totally useless, the repeater usefulness could be improved, barricades should cost less and be stronger, hovers are totally useless.
Like many said, one of the only reason why alien might look overpowered is because many players on Tremulous are total dick, and they want to win on their own, to be on the top of the kill rank and they forget all their team behind. They try to play human the same way they play alien : on their own. The problem is that you can't do that.
Just think about it : To kill a goon, you need around 6 MD shot. Alone, it's possible. If there's 6 good human together, in 1 second, the goon is down. It's the same thing for almost every units. I've found many other tactics while playing, but never put them really in practice because i rarely see players using teamplay, except a good clan I already saw playing, called CY (I love their mara dance ^^). Spread your damn humans and stop jacking off together. The more you spread, the more time it takes to the enemy to kill a human and reach the next one, and the next one, and the next one. Use diversions. Ask your teammates to use different weapons. You can have one guy doing close combat and baiting the alien, maybe 2, and another guy sniping with his Mass Driver from a long distance. Don't think you are god like, your name is not Flower (I can feel Iabz's reaction about this sentence hahaha).
Stop trying to grow your eDick up and play as a team. Turn Share OFF, people play like turd with this on. One alien do all the job, while others feed in dretch until the goon share. This is killing teamplay, it's horrible, and it's on on almost every server I played on.
Even in scrim, many people think they rock the shit because they can get 50 kills in a pub. It's not a good indicator to judge the skill of somebody. Kill whoring is easy, getting the real skill and the strategy is something else. Stop blaming your teammates. After a game, think about your strategy and how you could improve it. It's easy. Use your gun wisely, take the environment to your advantage, make the enemy think he have a chance to take the last bite out of you and blow him up with your fellow teammates.