At least it seems to be physically impossible to hit the hand, only a finger could be cut off.
It doesn't seem like you could cut off a finger, with the blade running lengthwise. No doubt it would hurt a lot, though. This is all a bit morbid. Oh, well I would trust Pd and the sensors in this situation (unless some jerk with a laser pointer was nearby). I see a little bit of wobble in the mechanism, where it's conceivable that even if the computer/sensors function correctly, you could still get hurt. When you rachet up the speed on this machine, that wobble (a resonance) is going to accumulate. It may be better to go with pneumatics for the knife's downward stroke, since there's fewer modes of vibration that way.
you know what they say...build a better knife-hand-chop-bot and they'll beat a path to your door