In many instances, we downgrade the maximum antenna length for a specific mount because of the way that the mount is most often utilized. Our main concern is not that the mount might fail but instead that the vehicle metal would lack the structural integrity to withstand the potential stresses. As in most instances, a little common sense will go a long way.
On some vehicle roofs you will find "U" shaped structural support beams. When you do, that is the place to locate the mount. You need to think "strength" and match the antenna and mount to the quality of the surface where the antenna is to be mounted. If the surface lacks structural integrity than it is nothing short of exercising good common sense to either use a lighter, thinner antennas, a mount with a larger base diameter or to add additional structural reinforcement where the mount is to be located.