I think that Riot will start to even out Sylas, he is still fairly new and Riots in house balance testing has never been good.
For starters, I think that they should pick if he is going to have either A. strong damage or B. strong sustain/mitigation. Because atm he excels in both. He has a very strong shield with 90%AP scaling and a second cast which deals ok damage. His W can be both an execute nuke (180 +55%AP or 270+82.5% if below 40%) and a huge self heal (190+35%AP or 285+52.5%AP if he is below 40%)
Large heals + large shields + large damage tend to make the champ problematic, as every time you think you have them, they mitigate you burst and heal back to full while chunking you. For me personally, I'd like to see power taken from W+E and added to his Q