I’ve never had the rails come off the car but I had a bike fly out of the Thule 591 carrier and into a field on a bend. The carrier was over ten years old and on inspection the ratchet strap teeth had rounded and offered very little security. Once they worked loose the design of the 591, with the open jaw that clamps the downtube, couldn’t hold the frame on it’s own. I have bought new 598’s and this have a wrap-around jaw and better ratchet straps compared to the 591.
For anyone with a 591, I would recommend checking the straps and making sure you have an “edge” on the teeth to prevent the same happening to you. As I could see the reason for the failure it doesn’t concern me carrying bikes on the roof, I just check everything is in good condition more often.
The bike was a Bird Zero and the damage was a cracked carbon handlebar, a bent reverb and torn seat. I got off lightly due to it landing in a field upside down. Ally bars and seat from the parts drawer, a brand x dropper and I was riding it at the weekend with no issues.