I will admit that all the times I have been hit, I have been 'lucky' in a sense that none of the stomatopods opened up there smashing hammers (exposing the sharp dagger) being hit by the dagger is far far worse...is the same force but the needle/dagger is being driven into the flesh the most likely outcome is it snapping off inside the flesh due to panic from both the stomatopod & the person struck, many species have inverted spines on their daggers to help them secure fleshy prey, this can make removing the snapped off dactyl/dagger from inside the flesh very difficult.
To make matters worse, stomatopods understand anatomy, if given the time (leave your hand in to long) they will 'assume' weak spots, on the hand for example this is the nails, the joints & the veins. I'm fortunate enough to of always been struck in a joint on one of my fingers, I did get hit on the finger tip once that scared the hell out of me cause if it hit my nail...god it would of hurt =X this anatomy behavior is extended from there day to day hunting activity, fingers resemble legs on other crustaceans, the joints are very similar which are swiftly targeted first as weak spots.
What helps to amplify all this is the mere fact that as humans we have many nerves in our hands in comparison to other parts this allows us to 'feel' better with this part of the body, this can be good & bad...