I had two wings fail as well although both only cracked the sheeting just behind and outboard of the plywood wing dowel plate. The first failed doing a snap at about half throttle. The second time I have no idea when it actually cracked as I had flown a full gallon in one afternoon and noticed the crack when I was disassembling for the day. I don't think it has happened very often since many people have many flights without incedent. Just our bad luck or our stlye of flying. I changed all the throws on mine to make it more patternlike versus 3D so that it lasts longer. I still enjoy flying it without the 3D capabilities.
Later,,,basmntdweller
It could be as simple as one wing builder not getting enough glue in a certain area and all the others builders doing a better job. That would be really hard for a mfr to trace down unless every part was ID'd by it's builder and GP's mfg was able to track it back to that individual and train them better.