Holy god, the Horde campaing is so goddamned awful. The AI will just keep spaming Freeze and Bolt the entire time, and pretty much kill your troops without any effort. The fact most of the time you don't start with Cavalry (which is the cornerstone of their army) makes the whole thing an uphill battle. Getting perfects there is a mind-numbing, exercise of frustration.
The Horde itself is really unbalanced too, as they can't fight defensively at all, and if you can't get the momentum to push for offense you'll pretty much lose the map instantly.
Otherwise, the game is fun, and I want to like it. But the Horde campaing just ruined it for me.