Some didn't break or bend axles, but many did. I recently broke one. I was a bike mechanic before there were cassette hubs, and riders of all kinds, not racers, broke them. Campagnolo Record hubs seem to have used good steel in theirs, as breakage was rarer in those hubs, but it wasn't unheard of. Cassette hubs are a big improvement in this respect. My aim is that henceforth, I won't build any rear wheels for myself that have thread-on freewheels since cassette hubs are so much better. On the other hand, as long as my wheels continue to serve me, I'll keep riding them. It made sense for me to replace my recently broken axle rather than replacing the hub or wheel.