My American cousin moved to Japan in 1969 after being posted there with the Peace Corps (Japan was still developing at that time).
He was enthusiastic, a "Japanophile", falling in love with many of the positive aspects of the country.
(I am a long term expat myself, though not in Japan. Much of what I will write below is true for many countries, not just Japan, as an expat. Some is unique to Japan. Also, do not misinterpret what I write as dislike of Japan. It is, perhaps, one of my favorite destinations for work or holiday.... but I would never live there)
Continuing on....
He became fluent in Japanese, married a Japanese girl, and had a son, who is half Japanese and a citizen. His language skill after nearly 50 years means that most Japanese people cannot tell that he is an American when speaking over the telephone.
And.... he is often very, very unhappy.I will note that he has been very successful financially. He owns aeveral houses and has done very well for himself.
Japanese society is incredibly closed to outsiders. There are also huge racial divides and predjudices. His son, who was born there and is half Japanese will NEVER be fully accepted there as Japanese. His wife effectively lost her entire family because she married a gaijin. The few remaining family members are just beginning to accept him after 45 years. His son was teased and bullied mercilessly in school because he was not really Japanese. The culture is unforgiving, punishes failure, and does not reward individualism. If you are the type who "sticks out" or likes to march to your own drummer, life in Japan may not be for you and may be very, very difficult.
This is truth.
This was very hard for him to accept. He did everything an expat is "supposed" to do in order to assimilate. He did everything "right". He later became very much disillusioned, and then angry, and finally accepting of his place in Japanese society. Note: This was a very difficult journey for him. He has become they "man without a country". He has been in Japan so long that he no longer "fits in" when he returns to the USA. But he will never be fully Japanese, either. That is simply the way it is.
I live in the Philippines. My wife is Filipina. My son is Filipino. The Philippines is far more open socially. Yet, I also know that I will never, EVER, be considered Filipino.
My comments.
This is of particular reference to those who don't want to bring their wives back home.
1:Yes things have improved a bit over time but it is still something I read about quite often.
2: WRT China and Japan and I assume Korea, most people split after a year or two, some having liked the experience and some not. If you stay, the next point is the 5 to 7 year mark. Very few people last much beyond that point, the only ones who do stay only because they have family who doesn't want to leave or they are stuck because of employment. The isolation of Japan, the feeling that the Chinese only want to use you for connections and English lessons to the point where some long timers hermit up. It seems to take 5 to 7 years for reality to set it.
3: Obviously there are exceptions. Duh. I am not sure, but I think Yohan seems to be one.
4: I suspect it would be much like that in most of Asia. You can gain acceptance if you have wealth by local standards of course but you will always be an outsider. Maybe the Philippines is a bit different but buddy doesn't seem to think so and that wealth effect would hide a lot. Thailand gets lot of claims of being a land of fake smiles and being pretty racist. India might be an exception because it, much like the Americas are pretty multicultural in a way most places are not, just don't speak Hindi to a Tamil

5: There is a huge difference between living in a place for a year or two and living there forever. There is also a huge difference knowing the language well and not knowing it at all. A lot of folks in Japan seem to say that learning Japanese beyond a survival basis is a bad idea because 1: They don't want you to and 2: the blinders come off and you realize what is actually going on.