the spacer bolts to the original hub. I have had no probs at all with these nuts.
i have once thou had a few nuts come loose on my right front wheel. It may have been from not been tight enough, but i mentioned it cos u asked if anyone had any wheel nut issues.
Mine would be a good jap brand i assume. All in all, these type of spacers, or blocks, are quite safe.
They are deemed illegal, as the distance from wheel to wheel is changed, hence outside the safety crash reports etc etc of the standard car, hence illegal.