I've spent the last decade living overseas. (Not FIREd, but as a trailing spouse for DH's work.)
I am quite sure this depends largely on personality, but living overseas can be far, far more challenging than I think most people consider. Sure, there's the language barrier and that can be a challenge and source of frustration. But it's also at least somewhat fixable. You can learn enough to be a functional part of society. That can take time, and be frustrating during that time. But learning native level language skills? Really, really hard and will take years, if you can ever manage it. For someone like me who thrives on the nuances of language and communication, that sucks, a lot. I can order a coffee and get my pizza with no onions and ask for directions. But I can't have a philosophical conversation. I can't carefully choose my words to make sure I'm not misunderstood. I can't thread the needle of awkward situations or difficult questions with carefully chosen words. I hate it.
And even if you master that, you will almost never master what I call cultural literacy. This is all the little things you learn without ever realizing you are learning, and know without even being aware that you know. They are everywhere in our lives. How to stand in line in Germany (hint: be right up on the person in front of you, or someone will see the opening and take it). That you pre-pay for a parking garage there. (If you drive up to the arm not having paid, you can't get out and the people behind you will be pissed and everyone will have to back up to let you out. It's embarrassing as hell.) That you don't wear even slippers on a tatami mat in Japan. Those are just a few, and they are the more concrete things. IME, I will always, always, always feel like a bit of an outsider overseas, and the more different from my home culture the place is, the more that's true--the larger and more difficult to bridge is the gap.
That said some people deal better with these things than others. That parking garage experience (which I actually avoided thanks to great info from friends when we arrived) is they type of thing that would have me feeling shame and humiliation for an hour. Getting screamed at in German (when I understood almost none) by the old lady in line behind me in line at the grocery store until you cried? Not fun. (She was expressing that one is supposed to lay down the bottles on the conveyer belt, not set them upright, but that was only figured out after much screaming from her and some crying by the bottle-placement-violator.)
And some places will be far, far more accepting of outsiders. Some places will also allow you to blend a bit better. (I stand out more in Japan than I did in Germany, for example.) That can be good, but also bad. No one in Japan ever mistakes me for a local and starts talking at me before I can explain I don't understand (or at least need them to speak very slowly and with small words). But they also never turn me away at the door with a "no foreigners".
Anyway, I think FIREing overseas can absolutely be great and is a viable option for many people. But the cavalierness with which some people toss it out there is misleading and does a disservice to those considering it. It's a huge decision, with implications I don't think most people can ever comprehend until they try it. But they can make sure they do as much research as possible and are honest with themselves and as prepared as possible for what is to come.