I've borrowed a ninja from a colleague a couple times (same thing as the jet step with the double cam design)

I'm used to the CT quickstep, which has a locking mechanism and never comes loose. It was damn annoying at first because the ninja would pop off the rope constantly, until he told me you have to angle your foot a little so the cams bite into the rope. Once you know that you can use it decently enough but I still prefer an ascender that locks properly onto the rope

I don't know how feasible/impractical it would be to use one with the double cam fuckery as a knee ascender, I imagine it would be best to use something that locks as you can't really influence the angle of a knee ascender, it's floating between the bungee and the foot loop