Yshaarj was the old god I wanted the least because he's the only Old God who will not receive any support beyond this expansion. He's a one off for a few months. The others will all continue to be tried for years. He's like the Paladin reborn quest in that way.
And, really, there aren't that many cards to go with Yshaarj. Some classes have 2 corrupt cards and some have only 1. The neutrals are mostly not being played. He's borderline good in 1-2 classes.
Corrupt has turned out to be very good in arena and not terribly common in constructed.
Unlike others Yshaarj is actually good, so no Yshaarj doesn't need more support to be viable.