So for a UA hero, is brandish/intrepid even worth it?

Soul Driver is WAY slower that IS/Brandish.

If you are only training/mobbing then yeah Soul > IS/Brandish

But for bossing Soul driver just plain suck.

That is, except for Zakum arms, depending on how you go about doing it, of course. Intrepid Slash has a horrible vertical range, so Soul Driver could be used in places like Zakum where Intrepid Slash will hit 2, maybe 3 arms at once, whereas Soul Driver can hit 4 easily.