yeah this is very true, although not universal. It's just worth at least researching ahead of time, or being prepared to try it out and return it if it doesn't work (which might involve significant shipping costs). Or just going with a US market device if that all seems like too much of a pain.
I bought a primarily chinese-market aimed device (low end and kinda old, so not directly comparable to a relatively new smartphone but still), and it does not have the correct LTE bands to get T-mobile or the VoLTE profiles for Verizon, so its definitely not an ideal experience though I have gotten some use out of it still. I haven't tried ATT but I don't think it will be better (and ofc MVNOs all piggyback off of the above)
not since the nexus one in 2010. Your data belongs on Google servers, not local storage