At this point, it's pretty much numbers. When she had the numbers to be able to build full tank, then dive into a teamfight and just smash everyone into oblivion, she was pretty OP. Once her numbers stabilized into normal ranges for a champion of her type, she fell behind rapidly.
One of the jobs of a tank is to either A) Dive the back line, or B) defend your allies and help to peel/chase/lock people down. Illaoi doesn't really have tools for that.
Illoai's kit is kind of dive-based, but mostly not. It feels like a case of the theme limiting the kit. The theme is all about the tentacles, and possibly setting up a good team fight with them, but other than that, she doesn't really do much. She has some sustain, some damage, but she lacks hard cc or a huge gap closer. Harsh lesson does give her a little bit of mobility, but it doesn't seem to be enough to dive the back line. Her numbers getting balanced means she's not going to be going all Darius on us, and she has no hard cc. Her test of spirit is interesting, but it doesn't prove to be a good lock-down tool.
The point is that Illaoi doesn't do anything that another champion doesn't do better.
If you want a dive tank, Garen does the job better
If you want a fighter/tank, Darius works better
If you want a cc tank... well every actual cc tank works better than Illaoi.
I feel like Riot can make her work, but these are the main problems with Illaoi right now.