A buddy of mine had a nice carbon Cannondale hardtail MTB pop off a rear rack and get busted to pieces on the highway.I had a bike on the back of a car during a blizzard. I salvaged most of the drivetrain but the chain was a goner. All rusted to high hell from the salt.
I did just remember that I tried to load my old steel gravel bike on the roof of my GX a couple of years ago when parked on a cross-slope. Not a good idea. I started falling backwards off the slider/tire, jumped off, and the bike hit the ground and bent the fork a little bit. Since Steel is Real, I was able to pretty easily bend the fork back, and the bike was pretty beat up anyway from thousands of miles of Ozark gravel. It's since been replaced with the Lynskey Ti gravel bike. Now I'm super careful to only load/unload on level ground. Since I often ride gravel in the middle of nowhere, I'll usually remove the bike on level ground, then park my rig off to the side of the road where it isn't level.
That's the only issue I've had in 15 years of using nothing but fork-mounted roof racks.