I've seen on another thread, on another forum that several folks report the design induces frame bowing. The additional weight behind the rear axles apparantly (after a fair bit of use) causes the frame rails to start bowing. It's my guess (read: wild stab in the dark) that the amount of flex depends on your SUV's frame/axle design.
[Disclaimer]
This was on a sportbike forum, and the question was geared towards carrying a sportbike. Not a dirtbike, which weighs in significantly less than a sportbike....