Here are my broken greasable u-joints broken for reference.

 

I can see where having a hole going through them can take away some of the strength. But what causes them to break. Cheap metal, bad wheel hop???

 

I was brake stalling on street tires Maybe 1500 rpm's, it let go. When I took the driveshaft to a reputable driveline shop, he said the greasable u-joints are very weak. Went on to say that sealed u-joints were the way to go.