I have seen them break, but not more than any other hubs really. The riders I've seen that had problems with them would probably go through almost any hubs just the same.

Granted I don't weigh much but one of my spare rear wheels was an XT 6 bolt that was abandoned my it's owner because it always had problems. After I overhauled it and properly tightened the locknuts it never had any issues.

If they had a problem out of the box it was that the axle locknuts were sometimes loose. I have seen riders go through freehubs, but again it's always that one guy who just destroys stuff, and not even big guys.

Also it's worth noting that it's a very inexpensive hub to repair, the freehub is the most expensive thing on it and it's maybe $40? to replace. It's also serviceable with basic hand tools, open end wrenches, cone wrenches and a large allen wrench. It's not exactly fun to service some high end hubs and very few have the tools to do it properly.