I've been considering one of those units, or at least something like it. My biggest concern is how they attach to the mirror. They're just held on by rubber straps? Does the unit shift around when you hit bumps in the road?
Is there a shakiness when driving? Yes
Does it flop around? nope
Longer explanation
Yes, it uses rubber straps and it does seem fairly secure. Of course I know how rubber bands age in the sun, so I have my concern of how they will hold up over a long time in the heat of the South.
But I think the more important piece would be the ability/stability of the original mirror to stay in place when adjusted. Some of the reviews/feedback I read talked about the mirror leaning forward (towards driver/cabin) due to a loose swivel/ball mount.
I originally had it over-top of my fatter Audi auto-dim mirror and wasn't happy with the fitment, so I swapped over to a normal flatter/squarer VW. I actually tried 3 different mirrors and had assembled/disassembled 2 or 3 more to get the smallest physical mirror with the tightest ball/swivel in the back of the mirror body.
When I get ready to install this in the other car, I will probably deconstruct a normal flat/square VW mirror by removing the glass and trim ring around it to decrease the total amount of weight on the ball/swivel. But by that time, I hope to have complete confidence in the performance of the device both as a screen and as a normal mirror.
As it is installed now (the screen and the camera) it needs to be adjusted differently for the proper screen use, or using passively as a mirror. I probably won't spend the time to adjust on this car, unless I decide to buy a second and leave this 1 in the MK4.