It's not hard for an Erebus to get his health per second up over 100. Somewhere in the 120-150 range even. That can be rough to fight.
Well, but consider that the only part of that 120 healing per second coming from himself explicitly is 20 HPs from poisoned blood. All the rest is just what anyone can get. Sedna is the one who really tanks out the self here, giving a potential 89 HPS (If I remember the aura/boost bonus' correctly). Erebus just gets 20 hps.
I really think that Erebus' strength comes from his flexibility. Mist, Bats, and Bite all make up a triumvirate of very effective solutions to various strategies and situations. Bite + bats = good at catching runners. Bats + Mist = ability to GTFO when needed. In my opinion, level 2 Batswarm is the most effective ability in the game. With mana, it allows you to both gank, run, and chase almost any opponent.
Now, I don't currently think Erebus is too strong, I'm just pointing these things out.