Double Uppercut: not so interesting?

I think the reason DU is a good skill is because of stun mastery. If you think about it, if you hit an already stunned monster with DU, both hits will have the critical opportunity, and at the same time continue the stun. Meanwhile, if you use FF, you only get 1 hit in and it's only a 60% multiplier. My belief is that it follows the same concept as Arrow Blow and Double Shot.

Yea, that works fine in 3rd job. The TS's main point is about 2nd job though, before Stun Mastery is gained.