I've never lost bot as Graves, just FYI.
I think he is EASILY as viable as Vayne and Kog. His burst allows him to push people off points, his mobility lets him kite the melee's, he gets innate tankiness in a game that's all about holding a point until help arrives.
His ult is an extremely long range interrupt, and his Q is strong AoE also for interrupting. Vayne can't interrupt an entire team capping, Graves and kog can.
He's not as strong as Kog in MOST situations, but is stronger than vayne. Kog is better against teams with melee/short range heroes that lack distance closers. Graves can deal better with teams that have things like Xin Zhao in them.