There was a change with Illaoi W with patch 9.18 that stopped her from being able to W and enemy even when rooted. This "fix" seems to have over hit her W.
Here you can see that she can no longer use W AT ALL when grounded. She isnt even able to use the melee version of the W. Before anyone says there is no melee version of her W, there is. If you pick Hail of Blades and do a long range W vs a melee range W and only the melee range gives you the auto attack reset while the long ranged one doesnt. This can also be tested with Sudden Impact.
**The fix: Allow Illaoi to cast W still but she cant W anything unless in melee range since W isnt a dash.** This will let her not be completely countered by ground when she is in melee range but still let the grounded effect have some of it's power.