Mag mounts are good except for 2 situations, IMO.
The first is pretty unlikely, however the more serious of the two, and that's if you ever stop super abruptly (as in an accident) that antenna could become a foil (fencing term?) and potentially pierce into something ahead of you. Again, unlikely, but a safety thing like bolting down your hi lift.
The second is the exact opposite direction, which is the tendency for tree branches to periodically lower and remove your magnetic antenna. It's almost a guarantee on Slaughterhouse, or one of the other wooded runs, less of an issue on something exposed obviously. But still, can be annoying.
that said, I've got a mag mount that I keep with one of my spare mobile rigs, that roof makes a dang good ground plane and it's a pretty sweet temporary setup. Just not a long term solution, IMO..