Validation branch. Human outpost Battlestation defense is pretty much useless, most it does, is scares away small raiders. Small alien fleet 1-2k easily destroys large human bases, despite 4+ battlestations. However, aliens stations require huge fleet to bombard. Aliens surface defense, at least by rating are like 3x more powerful.
So, my suggestion would be to better balance alliens vs human defenses, after all, allies have a natural advantage of needing various support structures, so their land stations would always be better defended. It's also not a good thing, that humans can only use marines to assault alien bases.