Crit buff is good, but we'll need to wait and see exactly how strong it is.
Crits, in ability form, seem to occupy the same frame, so they don't seem to stack?

Note that I specify in ability form, because the weapon skill stinger works differently. Stinger effects kind of exist independently; each individual stinger skill gets rolled. Then you add up the number of 'successes/activations'. Each activation throws +20% (additively) onto the special frame. And I have no idea how stinger and crit buff abilities interact.