Re: A side-effect
I took a look to have some specifics. As you were talking about American carriers, I looked up the bands in use. The one that most clearly makes my point is Verizon. Their main LTE band is band 13. Any device using their LTE network must support band 13 or it will stop working or drop to roaming inside the U.S. And not many other providers use band 13.
I did a search on handsets to see what ones were covered. Your flagships are there, of course, with Apple and Samsung highly represented. However, let's check a few less expensive varieties. I quite like Xiaomi devices. They run Lineage OS pretty well and are at a nice pricepoint. How many support that band? Answer: not many. A growing category of devices on the database I checked is the 4G feature phone. What if you want one of those? I found none. So I then checked Verizon's website to see what they offered for 4G feature phones. They have options. All of four of them. With prices ranging from $100 to $264. And the cheapest is a Verizon-specific variant of a device available elsewhere, but that variant only supports bands 4, 5, and 13. Those are all Verizon bands, and only band 5 is heavily used elsewhere. If you want to move to another country, you should hope that that country has a provider on band 5 and that you like that provider over all the competitors, because you have no other choice.
I then checked devices for the number of bands they covered. Apple's seem to be the best. The iPhone XS covers twenty one bands. But even that comes in four variants with a slightly different set of bands. For example, only one of the variants covers LTE band 11. If you're using your phone on the Japanese network Au, you'll need that one variant. Many other devices had four to eight bands. That may give you a few options, but not as many as you might hope.