Scary stuff.
The closest I have come to loosing one was when I had my long baidarka on my sedan with inadequate bar separation. No cradles, just sitting upside down on the crossbars.
I was driving down the icefields parkway from Jasper to Banff and passing the Columbia Icefields when a BIG katabatic wind gust leaned my car over. The boat swung several degrees off the center line, till the ends ended up pointing at opposite corners of the car. Fortunately I had it roped at the bow and stern with 3000lb test marine braid, and good cam straps in the middle so it stayed attached while I pulled over.

The only harm done was to the motorcyclist who had been tailgating me for a few kilometers, I suspect he needed new shorts from the way he got on the binders when the boat twisted. Scary to think of what could have happened.