Personally I'd prefer a rear view cam, mounted on the tail with an unobstructed view, and a display on the dashboard. Most cars have back-up cams now. But I doubt we're going to get it any time soon on our bikes. This seems like decent solution.
After having experienced excellent results with the Blueskysea brand in our cars, I installed the Blueskysea DV688 on my SR/F : https://www.blueskysea-inc.com/dv688-p00099p1.html (to be found on e.g. Amazon and AliExpress), for both dashcam/crashcam functionality and as a rear view mirror.
As a rear view mirror, it has a very wide PoV, covering everything in an almost 180° angle behind the rear wheel.
The disadvantage : due to the large PoV, everything which is more than 10-15 m behind you is more or less indiscernible on the rather tiny display (but is recorded on the storage card).
Actually, this helps for adapting : if you see ANYTHING on the display, it is close.
Are the cameras hard wired or can they be disconnected? I'm not interested in having a front camera as I already have one on my helmet.
How's the dynamic range? A common issue with rear view cameras is if you have a bright sky behind you the image can darken so much that it's hard to make out cars on the road.
How does the screen mount? I'd want to stick it to my wind screen above the dash so it would need to extend out from the mounting surface a bit and be able to be tilted up.
Is it fine to use it without a memory card or will it put a warning message up on the screen? It's fine if it's in the corner of the screen, just as long as it doesn't block the view.
Out of my head (SR/F currently out of order due to defunct 12V battery) :
1. Cameras can be disconnected , variable wire length, back camera can be mirrored, display emits a small warning sign and a beep (can be disabled if I remember well) when one of the cameras is not detected.
Not sure if this provides the best value for money if you're only interested in the back camera.
2. Dynamic range : it's winter over here, so not much experience with a bright sky.
Back camera is not oriented towards the sky neither, but rather level with the horizon. LCD display is a bit washed out (not to much contrast, depending on viewing angle), but recordings are fine (only looked at them when installing initially, as my main objective is rear view).
3. Display is currently mounted on the handlebar. I thought about mounting it above the dash (which provides a better view due to the helmet's limited downwards view), but at the handlebar it is a better fit for impact protection/aerodynamics/esthetics.
The SR/S may provide a better proposition here.
4. Memory card : it gives a warning message when absent/not formatted, but I do not remember where/how exactly. I see no value in not inserting a (limited storage) card : the device cycles the recordings (FIFO), and it provides some testimony in accidents too.
Is the device perfect ?
No (display slightly too small, display contrast could be a bit better, objects are small (convex PoV, as explained above), but it is not cheap China junk either.
Would I buy it again ?
Absolutely.