Ya, I just played 2 x erebus. I was reg, my partner was rook and I was just about to post the same question!
We got hammered.
Both went mega minions.
My mines basically did nothing and as soon as they got anywhere near the rook, he was dead. He had towers which did nothing.
I like the above poster who said, Erebus are very strong early on and wait till level 3-4 mines!
Considering that if they can outlevel you till you can get to level 3-4 mines (which by the way have no effect), you've already lost by about ten billion levels.
I also played Oak and the spirits really don't seem as strong but since I'm no expert on erebus, i can't comment.
Obviously, since I usually get beaten by erebus, I declare them OP! hahaha
Saying that, I'll play Darkness next time and play exactly the same tactic. And also a HP heavy UB which also seems pretty impossible to kill (so I also declare them OP - they must be since I have been beaten by them in the past).