"Illoai has very little counter-play."

Actually that's called counterpick :P

Yeah, Trundle has so much counterplay after he presses R on you

I don't think that it's a matter of lacking counter play, as much as it's a problem with how much value she gets for landing a E. Be it mid fight or in a passive lane, the amount of power and mechanics attached to her E is just to much.

Edit: To much on just the E... The amount of power in her kit is still up for debate (one i believe isn't in her favor), but just taking some of those interactions and moving them to other parts of the kit would make for a much more reasonable kit.

So let's say something like... " hitting a target or their spirit with Harsh Lesson puts a stack of vessel onto the target lasting for 60 seconds, at 3 or more stacks the vessel will spawn a tentacle around them every 10 seconds. Stacking up to 5 times. The if the target kills a tentacle they lose one stack of vessel, if the target kills illaoi, they remove all tentacle stacks."

You forgot

The common argument against Illaoi is not and has pretty much never been the lack of counterplay, the problem is her sheer damage output.