Genji's deflect ability in Overwatch is easily one of the more artful moves to properly pull off. I've played very little Genji over other players simply because I've already got my hands full with enough characters. I'm sure I'll swing around to learning Genji proper at some point in the near future.
However you don't have to play as him to realize the breadth of his abilities and what they can do. His Deflect ability is one of more controversial as there are very thin lines between what can and can't be considered a projectile and something has to be an actual projectile for it to be deflected.
For example: Hanzo's ultimate as the two dragon's can't be deflected BUT the arrow that comes before it spreads out can be. Check out this breakdown for the final word on exactly what you can send back and what you can't.