And, being on Verizon, my understanding is that I can pop in the micro-SIM from my 2013 Moto X and it will "just work", despite Verizon not officially supporting the 5x.
For that is a primary concern of mine. Due to circumstances, I'm kind of stuck with Verizon (in the U.S.). But I will never again purchase a phone whose updates are under their control. It seems to be either a Nexus that I can get to work on their network, or an iPhone.