I just got off of a round of playing and out of four games, three were 'standoffs' going to the max time limit, and one was overwhelming human victory.
On one hand the standoff indicates some balance in the gameplay, on the other hand it makes for very frustrating play. Generally my perception is that even good teamwork on the part of the aliens isn't enough to break through the often hefty defenses of the humans. And, when the aliens are laying it on thick, the humans can't really get out of their base to clear out the aliens, unless the aliens are dumb enough to lose all their evo points.
I don't really know the solution. Perhaps there should be a penalty to humans for staying in the base. If they had to get out into the map and collect food or something, then the aliens could at least 'starve em out'.
I guess the point here is that if the aliens have played it hard enough to keep the humans from being able to leave their base, then the game shouldn't carry on for 20+ minutes this way.
As for the aliens, perhaps eggs should be given a certain proximity to the OM. You can't build eggs too far from the OM and if the OM moves out of range after an egg goes up then their spawn rate goes way down. This may help the opposite effect where aliens go on forever even though the humans should be able to close the match.
As a final point, aliens need some kind of equivalent to a grenade to open up packed human bases. When they camp out in a narrow hallway, even tyrants can do nothing or very little. Maybe tyrants should have the ability to explode on death, spraying goo all over the turrets and clogging them up.
I don't want it to sound like I'm giving much of a solution, it's more a problem and a couple of initial thoughts.