illaoi ult is not the worst ult for sylas to steal.

Some ults are really dumb.

Vayne will make it so you become invisible when you use Q

I had a game once where Sylas was vs Jayce top and as soon as he hit 6 he just ulted the Jayce and never switched back so he doesn't get poked out anymore.

I'm serious this guy stood in that form for as long as he could. He backed, bought items and just walked back to lane still as Jayce.

Also there is a bug with Mordekaiser ult where Sylas can turn into Mordekaiser until he recasts his ult Looks pretty funny actually, the animations are totally fucked up.

https://leagueoflegends.fandom.com/wiki/Sylas says he doesn't get the W cooldown reduction.