well, the wal-mart ones have never shattered into powder nor more than two pieces, sometimes equal but more often not quite. It has usually occurred only at around 350fps from 1 metre away into a tiled floor. They do fly well too. I wouldn't want to risk anyone's health with them, though, so until further proof of safety/otherwise comes up I think it would be sensible to hold a moratorium.
Testing anyone? we could shoot them repeatedly into goggles from various angles.
For practice, they must be at the cheaper end so this may be a help to basement shooters. Check them out yourself with a micrometer, to satisfy they do no damage to barrels or hop-ups.