Sadly, I've been out of town and won't be able to play until tomorrow, but I'm thinking the backburner has to be overpowered.
Pyros have been played well for a long time without the air blast. And now you're giving them 100% crits when used from behind. When I play pyro, I love ambushing from behind, it always surprises people and while usually not killing them, at least annoyed and distracted them enough.
A crit flame kills someone in 2-3 seconds, right? So, basically, pyros can now sit around in a corner and just slaughter people coming through. 2fort sewers will never be the same.
Of course, maybe I'm wrong. I haven't played it yet.
But I do want to see the fun pyro vs. pyro fest that has to be going on.