Illaoi is "broken" against people that don't get her kit. If her tentacles were guaranteed to hit, she'd probably be the best champ in the game, but in reality they're pretty easy to dodge. Keeping her tentacles in check, dodging her skillshots, and not being an idiot and standing in her ult make her fairly tame.
I don't think we have a good sense of whether or not Yorick is good yet. He seems strong, sure, but my guess is that champs with access to basic cleaves to counter his ghouls won't have much trouble with him.