As in: can I get a phone in the USA and expect it to fully work in Europe/Asia/etc?
Simplified answer is: of course not. That would be a bit naive to think. Phones for the American markets are all adapted for each carrier, and each carrier got a very specific software inside their phones. I say “their phones” as I almost don’t see those as a “proper” HTC phone. Unlocked phones directly from HTC USA of course doesn’t have that carrier software in them, but: I would still say to stay clear of those since 4G LTE bands doesn’t have to be the same and that you will have NO warranty and so on.
When you get a phone you need to know just what market/network/country/etc it was originally made for it. Not where it’s made, but what market it’s intended for. Compare it with cars for the UK that got the steering wheel on the right where cars made for Sweden got the steering wheel on the left. Ok, a phone still looks the same but it doesn’t have to be the same.
I’ve written about How to know where your HTC phone was made. That will give you the hints you need.
Like: 4G LTE bands can be different. The software in it can be very very adapted, if it’s made for a carrier/network. Which in turn means that you might not have OTA updates. Which btw is the case if you get an American carrier phone (AT&T, Verizon, etc). Ok, you can get a manual download for some of those american phones but NOT for the Verizon ones. More on how to find out if your phone is branded.
So, in short: ALWAYS know what you buy. And, make sure that the phone you buy is HTC original.