Well, if you put it that way, SB still needs a nerf. If the only condition for it to crit is "you must shoot enemy back", then actual PvP would be very awkward. People tend to run forward, shoot, run back, etc. Moving around is constant, so if you just wait for the enemy to turn around and shoot him then you get a crit? Is that so? Well, that's definitely broken. Easy balance would be adding "no combat" status in the conditions, i.e. the perk applies if you shoot enemy's back and you are not in combat. Maybe even have to be sneaking. I haven't tried this perk out yet so I might have misunderstood it's working principles, but if I get it right, this is what I would suggest.