Mounting antennas to the rails of a rooftop cargo/luggage rack is doable as long as you keep a couple of things in mind.
Most, but not necessarily all roof racks are mounted to the vehicle roof with expanding rubber inserts. This prevents the roof rack itself from being chassis grounded. While that doesn't affect its purpose as a roof rack, it can affect your antenna installation. It would be best to check the roof rack grounding before you make the antenna system purchase. If the rack is not grounded then you will need to ground it in order to use a standard ground plane (GP) dependent antenna system. If you do not want to ground it then you will need to use a no-ground-plane (NGP) antenna system.
All standard ground plane (GP) dependent antennas require a chassis grounded antenna mount. If you clamp or bolt a bracket to an ungrounded rack you will have trouble tuning the antenna and performance will suffer. Test the ground. If the antenna, the mount and the coax has already been installed, disconnect the coax from the radio before you check for continuity between the mount and the vehicle body. Otherise, you will get a false reading back through the coax cable via the radios ground connection. If you decide to run a ground wire to the rack, use either a grounding braid or a wire that is 12ga or heavier and try to keep it as short as possible (Chassis grounding does not mean that the ground wire has to be affixed to the vehicles frame. Any common ground location is a part of the vehicles chassis ground).
Other things to keep in mind if using a GP antenna system … anodizing (on aluminum), powder coating and paint are all insulators. If your rack has an insulating finish (even if it is chassis grounded) you will still need to check that the antenna mount has contact with the grounded rack. Relying on a ground through the coax, as previously mentioned, is an SWR problem waiting to happen.
GP antenna systems, when properly tuned over a suitable ground plane, generate better field strength than do NGP systems. If you have available ground plane and want maximum performance, going through the process of grounding the rack and the mount would be worth the effort. However, if you are just a casual user who wants to talk with others in vehicles that you are traveling with or want an occasional highway check, you can skip the grounding requirements of a GP system and install one of the NGP systems on your ungrounded roof rack.
And let us not EVER forget that 2-way radio antennas MUST be tuned on the vehicle to maximize performance and to protect the radio from potential damage. Dare we say that an ounce of tuning is worth a pound of fried electronic components?