I would say if you just happened to use fanfir when your opponent is at little health but this just happened no to kill him/her, then going into tyrant rave is always a good way to finish things off, but other wise it just leaves your opponent too many opportunities if blocked. Sticking with BnN into a sidewinder loop is a lot less punishable.
Though this is a some what viable combo, it can still be severely punished if blocked but it is unburstable;BnN, whiff Fafnir>>FB Tyrant Rave >> Bandit Revolver.