Dont focus too much on amazon / ebay and so customer reviews, they are often fake / bought.
My issue with dashcams and backup camera use lie in them often dont having the guide lines in the footage and the abillety to flip the footage ( but that seem to change on some models now )
Also a dashcam ( front or rear ) are best placed up high on the glass, so if you drive a sedan than you have a blind spot directly behind your car where a child can easy fit.
Reversing cameras are often placed down low on the back of the car, so they cover from 0 to a couple of Meters behind the car, a rear camera on a sedan will only cover from 1-2 M and further out back.
But if you drive a car like mine with "no ass" then it can be done, same with a lot of SUV and 4X4 cars and vans, but my car are so small that its only my age and poor health that do i cant step out and lift the damn thing into place.
But i am old school so cameras or not i will keep doing it the old way.
This is the view out the back of my car wit the camera right up against the glass and so no heater lines in the footage, but as you can see i will have to tilt the camera down a lot to cover the area behind my car.