MundoversoTino_NoJust why?
It's viable against teams that don't know how to counter it, but most teams can easily have the scouts chip him away pretty soon. It's not as mobile, it's skill ceiling is relatively low, you have a very limited range, and it's pretty un-fun to play against.
Pyro is great for HL sure, but pretty lame in 6s.
(I'm assuming that's what you're talking about)