Speaking in general terms .... I see no reason a quality set of adapters aren't just as strong as the wheel center itself .... and I'll say they are likely even stronger ....
.... and they put no more strain on the hub / lugs than adding some dish to a wheel does as they only add an inch or inch and a half to offset (reduced backspace
Back in the old days, many guys ran 8 or 10 deep dish Cragars / Keystones on the GM products that had all of five 7/16
-20 studs (not even 1/2", like on Mopars and FoMoCos
). Big motors, heavy flyweels, some gear, and all in a car every bit as heavy or heavier than these MN12s .... I never saw properly tightened lugs / studs fail in all our time street roddin and days at the local drag strip on a street driven car. I had a '65 GTO with a later model RAIV 400, Hays 40lb flywheel in a Lakewood can, Munci 4spd, 67 Chevelle 12 bolt with 4.10s, ran mid 13s on Road Hugger L60-14 rear tires (just street tires, not drag radials, etc .... this was in early '70s
) on 14x8 Cragars always and never a problem. I know that those Cragars had more offset to the outside that these late model Mustang wheels on adapters.
Search for "failed wheel adapters" sometime. Don't consider those improperly installed, that happens with stock wheels as well .... seen it hundreds of times even on the Interstate as travelers had wheels break studs or simply come loose and they kept driving with a noise. But look for wheel adapters that failed.
My concern would be more towards shortened wheel bearing life
which is more likely any time you move tire center in relation to two wheel bearings sitting side by side on a stub in a control arm as found in an independent suspension setup.
JMHO ... it's worth what it cost you to read.