You have good points and I share your concerns... Yes, I have done my research.
This thread was intended for gathering info. from others on using this type of "flat" spacer and to share that info. It seems others here don't have much experience with these...
However, Nonstop, have you used these spacers from this vendor? I don't think we should assume this product is inferior, or comes from China, if we have not done our research. What I've found out so far is below...take it as you wish.
1) The material is 6061-T6 Billet Aluminum as mentioned in OP. I confirmed it is NOT cast 6061, and much lower tensile strength, with the vendor. Yes, they could be lying
2) These are made in the USA and the 6061-T6 is sourced from Canada, not China. Yes, they could be lying
3) These <.5" flat spacers are used frequently with tuner and high performance cars.
4) My 2014 WK2 has 1 5/8" of exposed (from the rotor) stud in the rear. There is 1 3/8" of exposed (from the rotor) stud in the front.
5) Stud type 14mm x 1.50 Jeep Bolt Pattern, BS, Lug Torque Specs
5) 1.5x the stud diameter should be used for determining proper thread engagement for safety. So 21mm, or .83" of thread engagement.
6) It is highly recommended NOT to use any spacer on the front wheels...flat or otherwise.
I see this as an informative experiment, which others could benefit from. For my scenario, I "should" be able to safely use an 8mm-10mm spacer and still have proper thread engagement, but I need to check the thickness of my new wheels. I may not even use them and definitely will not if I see my lugs loosening.