Basically what everybody else said. Illaoi's whole niche is dominating an entire area around her, but is weak if your opponent is even semi decent since they'll realize that they literally just need to take a few steps away from you to negate your entire kit.
So this basically leaves Illaoi in a very confusing state balance wise because her niche gameplay pattern is an extremely healthy and beneficial design and it makes her unique but is very easily countered, making it hard for her to be useful anywhere outside of low elo.
Same goes for every niche champion (Master Yi's reliance on autoattacks and glass cannon builds making him useful against uncoordinated opponents but useless in high elo, Swain's reliance on healing that is super oppressive but can be easily countered by just buying an item that low elos dont like to buy, Zilean's niche of basically being a free GA, etc.).
Illaoi would be fine if this game were not of a competitive nature, but since it is, every champion should be designed to have a variety of uses against players of various skill levels.
So this leaves us with a very difficult decision - do we want Illaoi to be a unique, fun, healthy champion with a clear niche, or do we want her to be a competitively viable pick?
This brings up a major underlying issue with champion design which I alluded to earlier. It's very hard to make a champion with clear, healthy gameplay patterns and still have them be fun and rewarding to play while ALSO needing to be competitively viable due to the competitive nature of League.
Maybe someone else that's smarter than me could pick apart this dilemma but I personally cannot.