Since the release of 2.0, I've seen aliens win time after time, with apparently little effort on their part. Things seem to be changing now as marines adapt their tactics to the new game. They are aggressively expanding instead of trying to fortify their base plus double res room, and it's working well. As the aliens become starved of resources they get weaker and weaker as a whole.
However, I expect we'll see a comeback eventually as aliens adapt *their* style to combat the marines more effectively, and begin to play as a cohesive team rather than just randomly roaming skulks picking off rambos and res towers.