[PHJF]

flash powder isnt worth it. energy cost is through-the-roof (74 at max default lvls), and, as maktaka said, flurry of knives is also crowd control. You don't really need both.

If you are like a corsair or (melee) brigand FP can be very useful as a follow up to any Charge skill, stunning groups before they can act, but again, you can just lob some knives at them from a distance to the same effect.

For reference, lvl 1 throwing knives + lvl 8 flurry of knives costs 26 energy.