Fiend is legitimately the worst character I have ever played from any mod. He's terrible. His pros do not outweigh his absolutely massive cons.
I don't know if the intention with him was to make an extremely difficult challenge character like The Lost, but that's what he is.

He loses about half his health with every hit and they turn into little minions that go do pitiful tickle damage to enemies and are useless. He can only take about 2-3 hits at any time in any room ever because of how this works.
In the Womb and onward, this is terrible. You need a ton of defensive items or have to be impeccable at dodging to do anything with him. His fireballs that sometimes spawn half hearts that very quickly disappear just aren't a real benefit. They vanish way too fast for him to be able to reliably get them. They WOULD be a lifeline, but they disappear far too fast. It would be great if they worked like Keeper

Fighting bosses like Mother is absolute hell. One hit and he's lost half of his hearts. At first I thought that if I mixed them, black, purple, black black, purple, black, that it would only take all of one kind of heart and stop at the next, but this is not so. It's always about half his health.
Then if I take one more hit, I'm on one heart. Now I can't be hit ever again or I die, while my pathetic, worthless, useless minions do like 2 damage a tick to Mother, who is very difficult to dodge already.

He's just plain bad. He stinks. The Lost is actually better, I legitimately have much better runs with The Lost and Tainted Lost than I do Fiend.
Fiend is worse than the worst character in the vanilla game.

All I've managed to do with him so far is defeat Isaac. That's it. After like 30 runs with this guy that's all I can accomplish, and I'm not terrible. I did absolutely everything with The Lost, Tainted Lost, Tainted Cain, Jacob and Esau and Tainted Jacob.

I know I'm going to be getting a lot of "skill issue"s and "git gud"s, but when The Lost is more rewarding and easier to play than this guy, there's a problem.