Plenty of other horses will buck you off. I'm free leasing a welsh cross and he took off bucking out of the blue the other day. I also had a buckskin (QH STB cross) and he was the most unpredictable horse I have ever met. You could never ever trust him not to throw a bucking fit, rear up, or spin out from under you... I was ran over by someone's arabian!
If you have a good trainer who is strong in natural horsemanship and good with building a solid foundation from the ground, you should not have any issues. People get into trouble when they try to cowboy a mustang and that horse turns out to be too much for them.
A mustang is not more likely to take off bucking than any other horse. As long as you take your time with training and build up trust first. If you can train a horse straight from the field that hasn't been handled, than you can handle a mustang.
With all the soundness problems in domestic horses now, a mustang is the way to go. They tend to be small, with good bone, and just seem to hold up well.