I don't let the thought even cross my mind. I've broke lots of bones in my days and recieved enough stitches on and off for the past 40 years to make a quilt. I'll be damned if I'm gonna let the fear of injury from a bow possibly breaking on me keep me from building them.
You just gotta do it right the first time so you don't break any, and know when to stop drawing it. If you hear a click, pop, tick, crunch, snap, or any other noises that a bowyer never likes to hear........just stop shooting it. Look it over real good and find out what's going on before it does break on ya, you may be able to fix whatever it was that is making the noise. But if it's broke in 2 there ain't much fixing that.
Course I ain't never found much that I was afraid of, except spiders and I'm learning to deal with them.