Bamboo backing survival might have to do with how the boo was harvested, seasoned, stored and prepared for backings just like selfbow wood. If you could harvest, season and prep your own boo the results might be better than 25%. Not picking on you Gordon.
I have only had one hickory backed bow break(cross grain on hickory backing) and have built many hickory backed bows, some with pretty crappy backing grain. I've had bellies fail but never had a sound hickory backing fail, no matter what the grain configuration.
Any sawn piece of wood will have grain violation just by sawing through the log. Also, there are more than one type of grain in a log. George Tsoukalas points this out in his description of a properly chosen board to be used for selfbows. I think the angle of the violation(s) is the more critical aspect of choosing the right board, whether for selfbows or backing strips but properly harvested, stored, seasoned and prepped wood(or boo) is also as important.
