The only thing truly powerful about the build is how early (level 6) it can be put together. But as far as endgame goes it's a great boss nuker but that's about it. The Rafa ordnance build I run fries bosses just as fast while offering top tier mob clearing also.
I think all four characters have a couple incredibly broken builds and I honestly wouldn't rate Bloodletter Vex in the top half of those builds. I have a max Vex and 3 crit knives (12,25,40).
