Over here in Denmark its pretty much illigal to hang anything off the windscreen, at least if you put it up high on the windscreen.
Things like NAV unit is okay, but you have to put it down low on the windscreen and in the mittle of it or in the far left corner beside the A-pillar
Judging from police programs on TV here it dont seem to be a law thats beeing used, you often see a cop pull over a car for somthing, and they never mention the NAV unit placed right smack in the middle of the windscreen.
And driving around you often see ppl with NAV unit and Phone holder in the middle of the windscreen, so it more or less look like they have a third illuminated A-pillar down the mittle of the windscreen.
Its allso illigal to have stuff hanging from your rear view mirror, again it dont look like its a law thats beeing used, you actually see that pretty often, and not only in the cars that is beeing driven by the 250.000 ppl here that originate from the south shores of the mediterranean sea or even further south and east.
The law in this matter is pretty vauge here, just like so many other things are.
In General the law say, anything that hinder the outlook for the driver is illigal, and this can be as little as a wire to somthing on or above the windscreen.
Even the up and comming heads up displays may be on the limit here, cuz even if the film you put on the windscreen is clear it is proberly illigal, the fact that there is numbers and maps reflecting on that clear film dont seem to be the problem to Danish law.
Factory installed heads up displays are OKAY, though i assume they work in the same way.
Denmark
EDIT:
I really think those sensor housings are stupid, and i dont see any reason for them, if ppl cant turn on ther vipres or light when it is needed, those ppl should not be allowed to drive.
And some sensore housings are huge and take up a lot of real estate on the windscreen, leading me to think that the ppl that allow them is in the pockets of auto makers, they surely cant have the safty of ppl in mind.
My little Suzuki dont have stuff like that, and i can hide a lot in front of and higher than my rear view mirror, what can be seen of my current 2 dashcams there can only be seen above the mirror, and that is okay in my book.