The problem with Illaoi is that to be efficient there must be 5 melee champions straight jumping on you, and in this meta there are a lot of mobile/high range champions, which you can't hit with your ult (Lucian, Azir, Kindred, Ezreal...). She also relies 100% on her E, which is a skillshot, has a long animation and has quite a big cooldown untill maxed with 40% cdr (which is still around 8-9 seconds) and missing it could be crucial. Also, there's a known bug that reduces the damage of overlapped tentacles (100-50-25-12.5...) which doesn't rewards her for hitting 3-4-5 targets with her ultimate, and of course, shows only 1/4 of her potential damage, which is already insane since she can out duel pretty much everyone in the game ( I know giving her 100% of tantalces damage would be broken [3k damage with 3 tentacles every 2 seconds without E] but giving her at least a 50% damage for every additional tentacle would be
a good start.