I have discussed 1/2x28 threads on .30 cals with TBAC's engineers, and the concern is not hoop stress or belling of the muzzle during firing, it's that with such a small thickness between the thread root and the groove, there's a concern that if you fell or otherwise smacked the can really hard, the high bending moment could cause a fracture through that small thickness. I'm not sure how much of a concern that should be, it's been a while since I did any fracture mechanics calculations like that, but I can see where they are coming from. Using a tightened adapter changes, but doesn't necessarily improve the forces the thread root closest to the shoulder would see in a situation like that.
Yep, that is the concern with small threads that leave a thin muzzle. I've tested that a bit myself, threading scrap barrels and applying some bending load; the concern of fracturing a 1/2"-28 30 cal at the thread root is valid and doesn't take as much load as some might think. It's one of those things, IMO, that's fine right up until it breaks.
The other concern about accuracy is more for precision rifles, a slightly expanded bore from turning the muzzle too thin is not helpful to accuracy, and that's why TB and some others offer 3/4" threads or even larger for some precision rifle stuff. For hunting rifle accuracy though I haven't seen it make enough difference to matter.
