The way I think about safety equipment is that the most effective kind is the one that you always have with you, that doesn't run out of batteries.
Maybe the idea of having a mirror near your face is a concern, but hey, you're already in a bicycle crash, it's not going to be great either way. Wear some safety glasses for the flying debris. The best plan is to prevent that situation in the first place.
I've seen it in a few places. I dug up some notes:
The following is from: "The Do's and Don'ts of Bike Helmet Care" at Consumer Reports (15 April 2017) https://www.consumerreports.org/bike-helmets/the-dos-and-don...
* "The strap and/or adhesive mounts could damage the helmet, especially if they’re installed less than perfectly."
* "the larger problem is the camera itself. A helmet is designed to have a surface without protrusions, and a camera represents a significant one. / ... "Anytime you add an external component to a helmet, you can change the performance of that helmet," says [Ian] Hall, [Consumer Product Safety Commission engineer]. "A helmet-mounted camera may become a projectile if it detaches in a crash, but a camera that remains attached could focus the forces applied to the rider’s head, increasing the risk of injury."
And from a Specialized helmet owner's manual: "Do not attach anything to your helmet, including mirrors or lights."
AFAIK, it undermines the integrity of the helmet in a crash. Perhaps it's worth the trade-off, but also having some loose stick of metal flying around my head in a crash is conceptually unappealing.