We broke my wife's Citroen AX's rear window with ours, but it was because it came with little bits of tube that were supposed to spread the load when behind a glass panel. That was fine, but they got trapped in the profile of the shut when opening and glass doesn't bend much.
The rack wouldn't be horizontal on the back of a cab as one leg sits on top of the bumper and the other on the boot lid. Mine was adjustable so you could make the prongs the bikes sit on lie at an angle to suit the position. The hooks over the boot lid were PVC coated so they didn't mark the paint and the edge of the panel is surprisingly strong. All that said it is still not the best way to carry bikes if there is an alternative available.
The rack wouldn't be horizontal on the back of a cab as one leg sits on top of the bumper and the other on the boot lid. Mine was adjustable so you could make the prongs the bikes sit on lie at an angle to suit the position. The hooks over the boot lid were PVC coated so they didn't mark the paint and the edge of the panel is surprisingly strong. All that said it is still not the best way to carry bikes if there is an alternative available.