So, how horn bow breaks? What is the most common defect? What is the most common source for problems. Sinew layer, core, horn, glue layers, materials, workmanship or something else. Was it total destruction or just minor problem?
The reason a horn has broken (you can select multiple choices):
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Heh, well basically all are bad workmanship!
I voted design, horn-core-joint and twist.
Sinew never breaks and when you learn to glue the backing properly, you never make mistakes anymore. And when the sinew is well made, cores don’t break either.
But… what is harder is to make sure horn is glued well! Also some weird twist can appear and angles for grip or siyahs can be all wrong. These can end up to a breakage when the bow sheds its string violently.
However, when the basics are learned, i think its easy to make successful bows time after time. Maybe even more than with wood only bows? Since we have pretty good materials for back and belly. 
Even for expert bowyers, wood backings can blow up and there can be some knot in the wood that is not seen.
Delaminations because of spoiled glue are a thing (never set the glue heater lower than 45°, or you’re growing bacteria like in a petri dish).
Faulty horn is another one that happens. It’s a bit harder to sieve out flawed horns, because the only good clue of problems is a sort of scaly skin on the surface. When these horns are sold in strips and you scrape them clean, they have a sort of whitish bright specs where the layers come apart. Sometimes they work ok, sometimes the layers just split apart.
Some horns have hidden defect that only show when you flex them.
It’s best to be quite rigorous and discard suspicious horn.