Post vets, we have ryad with her flippity-doo kturn antics, and marek making sure the crit counts. Now, in my experience, I certainly understand that actually getting Rex to flip a bunch of crits can be difficult, but when it does hit, it can be dandy. But mostly all I see around here is "Vessery or go home," notwithstanding the new pilots or x7s.
So first off. Why vessery? A free target lock, okay. It's nice. But am I misunderstanding? Can he just keep locking and rerolling every time he rolls, until he's satisfied with all three dice hitting/critting? Is that the secret? That would explain why he's so powerful, should that be the case.
But otherwise, considering a /D ion can strip another shield, flip that to a crit if needed, plus a primary attack to follow and offer another crit opportunity, why not rexler?