He's like the old Xin Zhao of today (post release, before he got nerfed to shit).
Pubstomps low Elo, doesn't work high because people know how to play him.
Hence the fact that Darius hasn't been played in a single tournament yet, afaik.
He isn't really a reliable damage dealer in teamfights though, if the enemy has exhaust or slows they can just kite darius 5ever. And he's pretty reliant on getting fed early on to really do anything in teamfights.