@ChrisA you and Brad make great points and I agree with you. I particularly follow Scott Johnston’s work with Uphill athlete (and now Evolv). SJ wasn’t absolutely against HRV btw, but that current methods of measuring them aren’t reliable (as we have experienced as well).
That said, I believe the main idea of the original question is whether this measurement is inconsistent when swapping watches. They probably somewhat are if there are differences in watch sensors. I mean, we can get different results on the same watch just by switching arms.