Are you sure about the one radio limitation? I wss under the impression that it supported most global bands. All 5 UMTS and most LTE bands are supported. I think that it’s more a function of carrier interoperability.International use of a cellular Apple Watch is not a contract thing but a hardware thing. Unlike most phones, which have radios that are compatible with the different cellular bands and formats around the globe, the Apple Watch comes with one radio that is compatible with the area it is purchased in. It's a simple matter of space as different bands require slight hardware differences as well as antenna differences. There's not enough room in the watch to cover the planet. So a watch purchased in the US will not work on cellular in some other parts of the world.
If the powers that be had been able to get their sh!t together thirty years ago and agree on a single mobile band and format, we would not have this issue.
I doubt that any of this will inform anyone sbout the diveability of the Ultra.