True.
However, same issue with the fender mounts. The additional drawback is that the cab is going to interfere, unless you space it far enough forward on the hood. However, it's still going to be physically lower than the roof mount.
I have mounted antennas like this on regular cab trucks. While not ideal, it does work and I had no issues getting the antenna SWR down low. The radiation pattern is no doubt funky, but again, same issue with the fender mounts.
Another benefit to mounting through the roof, as opposed to the bracket, is the underside of most NMO mounts are designed to be inside the vehicle, not outside and exposed to rain, road salts, oil, etc. Keeping the coax side of the NMO mount inside the dry vehicle will help it last a lot longer.