I dont quite think the value is there clearly enough in most decks
Hei Bai, Forest Guardian Hei Bai, Forest Guardian digs for a shrine upon entry and gives you a good payoff for controlling a few shrines
Birgi, God of Storytelling is busted, Halvar, God of Battle is perfect for equipment decks, and Tergrid, God of Fright is on the Commander Game Changers list
Returning a cheap creature with an enters or dies trigger to your hand makes paying two mana and two life seem like a small price to pay as you repeatedly drain lifedrain life, make tokensmake tokens, draw cardsdraw cards, make the table sacrifice creaturesmake the table sacrifice creatures, or worst of all, mill more cardsmill more cards to get better stuff back into your hand
you definitely want ample ways to trigger it on your own, but it's brutal in decks that can
Its hard to quantify an answer to that question, but Id guess that Gray Merchant of Asphodel is in the top five black game-enders ever