We are a 300lb team and have blown tyres off rims (with rim brakes) twice. Both on fast technical descents.
I am not an overly cautious downhill, quite the opposite actually. Do not ride the brakes. But with steep grades and a number of tight corners you simply have to work the brakes hard.
I use discs now and haven't had any trouble.
Really with my wife on the back I don't like the idea of cornering downhill at speed and wondering if the tyre is going to stay on because of all the hard braking that I have also been doing.
Fortunately the two times we blew tyres were on straight bits and we managed to stay upright.