They’re okay.. Great for hardtails (not small ones) as the arms fit through the frame easily. It’s trickier with a FS depending on frame design but with a bit of fiddling you can get them on there.
I had a Saris bones 2 and used to change the arms around (inboard to outboard and vice versa) for the two that went down to the bumper and the two that the bike rests on, in order to get my FS on.
Also best to put a couple of clear pads (helitape type stuff) on the bumper as the rubber pads will move/rub slightly and damage paintwork.
Another consideration is a light/numberplate board as they tend to obscure the back end of the car.