The rear view module's results seem to vary depending on the situation. Your results seem to worsen the longer the vehicle is as you pick up more vehicle interior, this wouldn't be as much as an issue if the module had an adjustable zoom to compensate for this. As someone else pointed out, head rest design and placement can have adverse effects on what the camera picks up, and having rear passengers almost makes it useless (particularly if they are tall people).
That being said, aside from ease of installation one big advantage the rear view module has over the rear window module is it allows you to have a dash cam with a rear view that is easily portable between vehicles (which is some what helped by Nextbase's Click and Go feature). Almost all traditional rear facing cameras are semi-permanent with installation, with adhesives and tucking wires into the vehicle trim. While the view may not be as good as a traditional rear cam, I think not having to buy and install multiple rear cams if you are moving dash cams between vehicles is worth the tradeoff.
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