[2020] Which champion’s R would you consider it a waste of Sylas’ R to steal?

I was watching a Sylas vs Illaoi matchup and it had me thinking about the R’s. If Sylas stole her ulti when he uses it, would it spawn tentacles that slam? Obv he can’t gain the buff to the W like Illaoi does which got me Thinking it might not be worth it to take her ult? Which then led me to think if there were any ults that are just downright bad for Sylas to take?