The reason that they cannot be used with the spoiler is that the way the carrier fits means that the weight is taken at the top and these top mounts usually just hook round an edge, in this case the (likely) glued on rear spoiler. I tried this with an older car and although I got the grip round the hatch metal, the weight still distorted the spoiler, the carrier was never used in anger fitted to that car. I have tried a number of these and find them very unsatisfactory, they squirm around creating pressure points on the paint and glass, in my mind a recipe for damage and/or falling off. I have bitten the bullet and bought a proper tow bar mounted carrier, the tow bars are less expensive outside the dealer network of course. The end result is safety and security, bearing in mind that a tailgate mounted unit cannot be locked in place as it is only on with straps. Single electrics on the tow bar are sufficient lessening the cost too.