I use an eyeglass mounted mirror (I use the "Take a look" mirror, I highly recommend it), and the only time I was in an accident with it, it popped off my glasses without gouging me in the eye. Any up/down motion of the mirror is going to make it pop right off of the glasses, lateral side-to-side motion will likely just bend the metal shaft. If my face hits the ground with enough force to shatter the glass mirror, I'm already looking at some significant facial injuries so I'm not too concerned with the mirror making things worse -- small shards of glass sound only marginally worse than smacking my face on pavement.
It would take a freak accident to get it to shift around such that it would poke me in the eye given that the end of the mirror shaft is behind the bony part of my eye socket. Plus, the polycarbonate lenses of my sunglasses shield my entire eye plus a bit of wraparound to the side, giving me some protection from the mirror. That doesn't mean that there's no risk at all, but I feel that the enhanced safety I get from the mirror is worth the tradeoff of increased risk of injury in an accident.
I've tried some helmet mounted mirrors but found that my helmet shifted around enough to be annoying, my sunglasses don't shift, so the eyeglass mounted mirror is always in exactly the same place.