I've played a lot of Yorick since he came out. I really enjoy his kit.
I think his power curve should be smoothed out in general. He is really brutal in the mid game, even if he is behind in gold. But he falls off like a brick late game.
It seems like he is supposed to be a dedicated split push champion. But I would really appreciate more consistency in team fights. So much of his damage is locked up in his ultimate % health damage, but the maiden goes down in about 3 late game ADC autos. The ghouls are also very squishy, like you mention.
Unless he is fed enough to consistently 2v1, he can't even really split push very safely past the laning phase, since he has no escape. It often feels like I am a glorified wall-bot in late game team fights. Just trying to trap any divers.
Maybe enemy (or ally) champion deaths could automatically summon a ghoul. So he has some access to his minions (outside of his ultimate) during team fights.
Or do something to make them not so likely to die from collateral damage in fights. Right now, it feels like people don't even really have to try to kill them. They're just going to go down on their own from miscellaneous AoE.
In general, I don't think he's that bad of a champion. He doesn't need a massive buff or anything. But, ya, like I said at the start, smoothing out his power curve would make him more fun to play as and play against I think.