Jokum: Geezer here. In my experience as a crossbow maker for 35 years, I have seen ONE metal prod broken, that actually injured the shooter. A pal made a lightweight (70lb.) draw aluminum-prod crossbow, made with a leather cover over the prod. Later he chose to remove the leather cover, by cutting loose the stitching along the back, and cutting it free just outside the bridle-block. In doing so, he scored the prod all the way 'round. Unlike steel, which requires a substantial notch for crack propogation, aluminum is more like glass... a little scratch across the 'back' will do the job. So Rick took his bow out back and shot it about three times. On the third draw, the prod broke one one side, the end whipped back and struck his hand. It drew blood, barely. He went in the house and applied a band-aid. We replaced the broken prod with a new one and he went on shooting his bow, but he was pretty jumpy about drawing it for a few days.
That's the worst I know from personal experience. We've had several aluminum-alloy prods break over the years. To my knowledge, we haven't had anybody else hurt. That isn't to say it hasn't happened, but I haven't heard about it. I believe a few steel prods out of Alchem's first batches broke, but don't have any numbers. If there had been serious injuries, or deaths, I think I would have heard of them, but maybe not....
Then again, I'm making fairly light-weight bows, nothing over 200 lb. as a rule, and all of those have used new, purpose-built prods. Recycled leaf-spring prods, or prods drawn close to the limits of their ability might make a different tale.
I would recommend fitting a 'sicher' strap down the back of the prod on stronger bows as a matter of course. Geezer.