They've been around for a long time but I've never heard of anyone who's used one. There might be a warning right there or it could just be they're not popular as an idea.
As with any camera, I'd be less concerned about the mirror, provided it wiil fit over your existing mirror, and more concerned with the camera specs. The mirror is just a screen. Ultimately, it's the camera quality you need to worry about and for that you need to know the CMOS used, the chipset and independent tests / video day and night on YT or dashcamera forums would probably help with a decision. I never trust vendors or manufacturers website for sample as they nearly always look way too perfect.
One reason they might not be popular is any video displayed on the screen will likely make it very difficult to see anything reflected from behind as the 2 images will merge. Same with watching footage back. I Would have to question the legality in some jurisdictions as well because of this where rear view mirrors are compulsory because if the image is displayed whilst recoridng, then it might create an offence if the rear view isn't clear. This might be a reason why they're not popular. If rear view is interefered with by the video image and video playback is affected by the rear view reflections, you'd have to question how useful they really are. Don't know if this is the case as never had one!