It is an overlooked weapon for two reasons: Sienna has only one anti-armour/super armour ranged option, being the bolt staff, which in itself is hardly ever used, and you don’t want sienna to be on the frontline, at least neither as Pyro nor as BW.
The flamesword does complement the bolt staff very well and I do think it is a very effective combination, solely for the reason that the flame sword makes you virtually untouchable during hordes or ambushes where you cannot position properly to use your staff, and it absolutely deletes hordes. It is, imho, one of the best CC and horde clearing melee weapons in the game. The sweaps are wide, it cleaves very well, it knocks down mobs, it has high attack speed even on charged attacks and it deals significant damage over time. If you put swift slaying on that thing as Pyro and have some overcharge running, you can become quite the force to reckoned with.
It could use some tweakins and would benefit from reliable anti-armour attacks - and I’d also like to see the initial clawing attack on charged to deal more damage (and an overall slight damage increase wouldn’t hurt).