In my experience, the issue with the thinner wheel adapters has to do with the amount of material that is remaining under the conical seat for the lug nut. All of the force from the lug nuts is trasmitted through that conical seat, so the thinner it is, the lower the factor of safety. Due to the relative thinness of the adapter, lug nut interference with the wheel is a possibility, so it's a matter of gaining enough room for the wheel to clear the inner set of lug nuts without sacrificing more material than necessary.
For comparison, a cast C6 5-spoke wheel has ~5/16" thickness as the lug nut seat since it's pocketed on the backside. The depth that the lug nut sits in the adapter will be determined by the diameter of the 60 degree countersink, however even the shortest lug nuts will still add 1/2" or more, so you'll typically need reliefs in the wheels to clear standard, short, bulge-nosed lug nuts with any adapter 7/8" thick or less.
Tobin
KORE3
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