Like with other weapons that have high critical damage bonus, as your character gets stronger and you min-max critical damage bonus, you become able to kill many enemies in 1-2 critical hits. And this is primarily what keeps crossbows, pistols, and certain melee weapons as viable alternatives to psi or ARs, albeit still weaker.
The problem is, if you kill an enemy on your first Flurry hit, then you don't get stacks from the second or third Flurry hit. It's pretty awkward to be getting 1-2 less stacks per turn because you've built a strong character with very high critical hit chance. It incentivizes gimmicky gameplay like having a low quality off-hand sword to build up stacks with before switching to your primary sword.
I think there's a simple fix. Change Flurry to provide an additional stack if it kills the target, but also, make it so a single Flurry can't add more than 3 stacks (this way you'll still need at least two Flurrys to get max stacks). Alternatively, this could be implemented as a specialization.
I think this is a fair and relatively minor buff, that only starts to have some impact in the late game, and helps a build that is clearly far less strong than psi or AR. But I think it is needed because Flurry is the primary reason to use swords and there isn't another build in the game where feats like Critical Power and Cheap Shots come with a notable, objective downside.
