I've got one that mounts to the spare wheel and I've gotta say, I never feel completely comfortable using it on greenlaning trips (although I do). The saving grace of mine is the straps that hook over the top of the back door when the bikes are on, and these take a good portion of the weight off the spare wheel mount. I do sometimes wonder how long my back door hinges are going to last and I keep checking them for stress marks but it's all good so far...
Jim.