Just picked him up and he is a really stong pick imo. Got huge amounts of CC, very good damage after titanic+sunfire, very very tanky and has great engage. I got 59% winrate with him over 41 games and 3.64 average KDA with him.
Cause noone picks Vi an everyone knows most important thing in this game is ViSion
I know I'd love to see a mecha Sion rolling down the tracks into a team fight on stage.
Slowness of a lot of his CC I suppose
Because he is super easy to outplay, and contributes nothing to a team fight except sort of zoning but not really zoning.
