I personnaly do not think base mana is a serious contender for comparision as you can buy cheap mana potions and scaled helm right away which makes Bite that much more efficient the longer the fight lasts. It's true Sedna will be a pain to Erebus, but Sedna shouldn't be the only solution to him.
Well, so can the other team... Bite doesn't do an amazing amount of damage at all. It's just a debuff/self heal. Sedna can outlast an erebus. QoT can out-last an erebus. Oak will outlast Erebus. Pretty much ANY general will counter him, because they have much more support to mitigate his damage. He is literally an assassin/general hybrid. He is most effectively played as a general to support, or as a direct counter to ranged demigods. This is what I have observed thus far. I play exclusively premades as well, and I've yet to be contradicted. My stance can possibly change, but so can yours. Thats why I again, argue its too early to judge him.
He is also THE squishiest melee demigod there is. He has to rely on either A) his bite, or his minions. No one has yet to present a case where statwise/skillwise, he is SUPERIOR vs. other melee demigods. The only real melee demigod he can out-play (keyword, OUTPLAY) is a rook, because mistform/batform can dodge a slam and force a Rook back with no real way to nuke an erebus. One hit from slam though, and erebus is at HALF health in any equal situation. ANY stun in a team game, and there will be way too much damage for him to simply 'bite' it back. The game doesn't always play out 1v1, and people forget this.
It's not something absolutely game breaking, but it does tend to make Erebrus better than he should be against certain builds.
Why is this a problem? From what I have seen and played so far, he can very easily be built to counter CERTAIN demigods. Every demigod can be built to out-play others. Bite still doesn't make him superior vs. other melee demigods, like UB, Oak or Sedna.
It's called counter-play man. This is where skill hasn't been factored into the whole, 'overpowered' equation.
As for the stupid mana argument that everyone else has been making, I'll grind out real numbers when I feel like it. They won't mean anything though, because you would have to put ALL of a demigod's skills into play, rate the skill efficiency which submerges into the realm of opinion, then put every demigod against eachother, which ultimately does no justice to a balance for a TEAM game. This in itself proves 'mana efficiency' a useless argument.