Sylas' Shield Should Not Be Magic-Only
I understand if Sylas is intended to be a counter to AP champions since his lore is about him stealing magic, but making his shield magic damage only is a bit too much, I think. Especially because Sylas' ultimate allowing him to steal the ultimate of any champion in the game has much more limited use when he's funnelled into being a counter-pick for AP heavy teams.
Rather than be magic damage only, I think the shield health could be a bit lower, say 65/100/135/170/205 +90% AP while blocking all types of damage, and grant him bonus magic resistance for as long as it persists, around 20/25/30/35/40.