Sylas's Kit is way to strong to justify his ult.
Plain and simple. He already has enough tools to safely 1v2 people who both have ults with just his base kit.
But he can also take any of their ults which are specifically balanced around them and add them to his purposefully overloaded kit.
His identity is supposed to be someone who's strength is dependent on the enemies strength. That's fine. But the fact of the matter is his base kit makes him not even need an ult. Which is kind of shitty.
If you have high impact ults? Too bad he uses them to shit on you.
If you don't? You have no way to fight him because he is balanced around not having ults. Oh and he can still take whatever you have.