I haven't broken very may bows in the making stage, 3 or 4 breaks but at least that many badly hinged out of 156 bows so far. Most failures were cause by me getting in a hurry, a few by a poor stave choice.
The other end of this question is how may failed somewhere down the road, blew up, popped a splinter and such. Most of my failures in this realm were from 1 to 5 years after the bow was made. My guess would be 10% for general after market failures.
I have a failure rate of about 100% for bows shot heavily for over 5 years with between 100K and 250K shots put through them by customers, all eventually popped a splinter. Osage is great stuff but it does have its limits.