Too effing frustrating later on.
When you said that the Alien campaign would be "harder", I wasn't expecting that to mean that I would be fighting against opponents that had half again as much health as me, who seem to always take all the weapons, and who never, ever miss a shot. Other unexpected features included my teammates constantly standing between me and my target, and missiles, grenades and bullets that would pass through their intended targets and just keep going, doing no damage. I loved the human campaign, to be honest, but could you maybe look into balancing the aliens? And fixing your gameplay?