I never expected mine to be a popular position. I've no idea if an engineer designed these spacers or not. I can not imagine an engineer not doing an extensive analysis on them if he or she had known that they were going to be used on the street. This comes back to does the mfg say that they are DOT compliant and safe for street use? I highly doubt it, but I could be wrong.
For their original application, rock buggies & what-not, they're fine. No issue with their use there at all. I see some advantages to using them in that type of application.
Whom ever originated making them this way may have done the homework to make them live and be safe for street use. I highly doubt that this is possible, but it might have happened. That is no guaranty that those who copied the original design didn't make some simple change(s) that take them into the unsafe zone.
Say the design is only safe when made from 7075, but most make them from the cheaper and more common 6061. That is like taking a design intended for 4130 Chro-Moly and making it from 1018 mild steel. There is no reasonable way that the consumer could tell the difference, until the part(s) fail. Those with spacers who find that their lug nuts frequently need to be re-torqued should be particularly on guard. Something is moving or distorting for that to happen. Not a good sign.
The fatigue life issue with the spacers is directly related to the stress concentration. Low stress = long life. High stress = short life. Lots of cuts, pockets, and notches = high stress.
Wheels are designed to be comparatively low stress components. There is NO way to make these spacers low stress components, can only minimize the stress and find a material that gives a suitable life.
Perhaps the mfg's ought to take a clue from the OE's? They make similar spacers for the front of dually trucks. Those spacers are well designed and made from a material with practically an infinite fatigue life.
I've 'known' Grim on a couple of other forums over the span of several years. He does not strike me as one who is lax in his maintenance, yet a spacer failed and set large tire free at speed.