Illaoi is an extremely oppressive champion to fight not because her kit has little room for counterplay.
dodge q, e
walk away from her r
But her e is an extremely oppressive ability, and with corruption potion acting as an extreme mana buffer for ability based champions, her ability spam becomes excessively oppressive.
I recently played a game where i was up 40 cs, 2/0 on darius and attempted to all out illaoi after clearing nearby tentacles, i ghosted towards her and i baited her e, r combo so she wouldnt be able to to kill me. And as it goes, I was wrong, e did 1.2k only dropping me to a whopping 10% hp. And because i opted to walk out of it. I was slowed.