A couple of comments on this subject. We've never seen a stud failure due to this particular problem, because we've never seen this interference problem before.
All OEMs have radius/chamfer specifications for the bore in the wheel and for the base of the hub pilot. If wheel spacer manufacturers simply followed these, this problem could never occur. The wheel spacers that Chris shows in the video are incorrectly designed. The radius at the base of the hub pilot should never be this large. A small radius here does not create a stress riser, because the only stress on the wheel spacer hub pilot, is when the wheel and tire are hung on it, before the lug nuts are tightened. Once the lug nuts are tightened, the chamfer between the lug nuts and wheel stud holes centers the wheel and all load on the hub pilot is zero.
The only possible issue here is that if there is a chamfer on the back side of the thru hole in the spacer, you may need to put a larger radius at the base of the hub pilot on the wheel side of the spacer, so that there is enough material thickness. This effectively puts a limit on how thin of hub centric wheel spacer one can build.
The types of wheel stud failures that we have seen are caused by a similar issue. The hub pilot on a hub centric wheel spacer has a particular length and OD. Some wheels (Enkeis are the worst about this) have a smaller thru hole in the wheel center, designed to fit their cap on the outside. Inside this, on the hub side, is a counterbore for the hub pilot. This hub pilot counterbore is not deep enough to fit on many Ford 5 x 4.5" applications, without the hub pilot bottoming out at the end of the counterbore. When used with hub centric wheel spacers, the same problem exists. The solution is to file about 0.060" off of the tip of the hub pilot on the wheel spacer.
I believe that most 5 x 4.5" Enkei wheels are designed around a Toyota application, which is why they have such a large counterbore in them (>2.775"). This application must have a shorter hub pilot than Ford uses on a Mustang. Enkei should be maching the hub pilot counterbore deeper, so that it is compatible with Ford axles and hubs. I've brought this up with them on a couple of occasions at SEMA. They don't seem to care.
Chris' paper test will work to test for both cases of improper wheel spacer fitment.