I would add from experience that getting repeatible mix quality with resin materials is very difficult, our resin work in the defense industry was highly variable.
A number of missiles with fiber wound resin shells dis not perform well and had to be culled from the fleet. (Mix was inconsistant for a while, til we learned how to do it.)
Great point.
Of course, you can also get non-homogenous metals as well, but voids, inclusions, grain problems, and other problems can be found with 100% radiography. I was in the power industry for a long time and radiographing boiler tube welds was very common.