If you check his winrate, which is between 30 and 46% depending on the role, you see that he is absolute garbage. And honnestly, I agree: he is. I stomped him in every lane Ive seen and only once have I seen one with a lead... And I stomped him hard after that.
Sylas is: bad at getting a lead because he doesnt do that much damage, hes more like an inconvenience
Sylas is: bad at keeping a lead because hes a melee champ who gets shutdown easily with some control
Objectively, he is weak. Now the real question is.. With a buff, will he be better? The answer is no, this champ is doomed to be unplayable because of his stupidly made ultimate and Ill explain:
Every champ needs to be around 50% winrate to be balanced. That means they need some weakner match ups against which they have a 48% winrate for example and some against which they have a 52% winrate (and that balances to 50%).
But Sylas? His winrate swings extra hard without control. Either he has a 30% chancee to win the game because the ennemies have champs with terrible ultimates (for example Illaoi whos ult is useless on Sylas) so all he really has is a Passive and Q, W, E or a 70% chance to win because his ennemies have amazing ultimates (like Karthus).
Basically, he is inherently flawed because of that ultimate. He either loses with 30% or wins with 70% and that gives him an average of 50% so if he LOOKS balanced with a 50% winrate, it means if you only use him as a counterpick (when you see the ennemy team, so as a last pick), you winrate is going to skyrocket to fucking Pluto and he will be unbalanced.
That means Riot will never be able to balance him so hes going to have a negative winrate forever, sadly. (unless they do a baddie in which case he will be op)