Alright, first off let's go through Sylas' kit.
Passive: Basically an AoE no CD Lich Bane effect.
Q: A decent AoE, long range ability that does alright damage, slows twice and has a very short cooldown for what it does. Also costs barely any mana.
W: A pseudo-execute, also grants bonkers healing. Resets AA aswell so it will proc passive instantly.
E: Dash+a pretty decent Shield resets AA coz why not, allows him to recast for a hook which stuns and knocks up the enemy. deals avg damage.
R: Can steal ultimates, at the low cost of 75 mana (opposed to 100 like nearly every other champ) then can cast it later at 0 cost. Stolen ults scale with his ult level and stats (meaning ults with high ratios tuned for the intended champion can be abused). Also any ults that have AD ratios will have those ratios converted to AP just so that his ult choice stays varied.
So he has:
range with Q and E with relatively short CDR - so you can't punish him because of his melee
AoE clear with Q and Passive - so he can clear waves easily and you cant just shove waves against him.
a Shield and healing - so his sustain in fights is really high.
Multiple AA resets - in case his damage wasnt high enough, he can reset AAs to do more damage that way and proc Passive more.
Acces to a variety of ultimates - he can pick ults that fit him at that time in the game, also plenty of ultimates that will have much better damage than intended because of higher ratios on non-mages.
So I really dont know why this champ needs everything in his kit. The one thing he doesnt have is invisibility (except if he would steal an ult that grants invisibility like Rengar's or Kha'Zix's)
If anyone who does have an idea or a rioter could explain to me why his kit is so diverse and imo just straight up overloaded then that would be great.
Thats not even talking about the fact that he is extremely hard to balance due to his ult btw.