Illaoi is a very balanced and manageable champion
Until she ults. Then she does freaking ridiculous amounts of near-unavoidable damage without even trying and beats the living crap out of you. Seriously, I have no problem dealing with her in lane, but once her ult comes up, she just slaughters you with it, multiple tentacles beating the living crap out of you and her spamming her W over and over again. The ult itself has a decent damage, but the issue is it has so many passives it's not even funny. Summon up to 6 tentacles, tentacles are immortal, tentacles swing faster and W has a massively reduced CD. The level of power crammed into that move is just crazy.
Imho she'd be fine, if it wasn't for her ult. It's just way, way too strong.