He ain't any different in terms of power against Irelia, Akali, Yasuo, Syndra etc... He's easily countered by any range champions and Lucian will rekt hell on him if he goes Mid. Syndra easily can win this lane or safe farm under tower if he's too fed.
"BUT, BUT... I played against Sylas in Mid lane as Katarina and lost very badly." I'll let you figure it out.
"His Q is too strong." It's really not. Try side stepping once in a while.
Here's an idea. Play champions that will over power him. You know, champions like Trundle, Darius, Riven, any ranged champions, Irelia, etc... The list goes on and on.
"He's too damn tanky." No he isn't. If you want to talk about him being able to heal, try playing against him Mid lane as Aatrox. You'll win more often.
"His kit is overpowered, it has heal, shield, and dash." Am I missing something here? You forgot about Diana the AP assassin with heal, shield, dash, slow, and knock up? What about Pyke who even has speed boost to escape + invisibility. Oh hey Riven, you have a shield and dash and your kit allows you to build life steal on top of it.
Sylas is an AP assassin. If you play squishy or try to go against him as one, be prepared to die because he's made specifically to counter you. It's no wonder you lose and your idea of him being "overpowered" is really just you being a noob.