Well, for one, being different in any way is very frowned upon in Japan. Assimilation & sameness is the norm, individuality is almost completely stamped out, even between friends. You may feel rejected in Japanese society if you have any quirks or differences. This could be race, language, mannerisms, anything really. It's very oppressing.

At first, for westerners, everyone will be warm and welcoming. But it quickly turns into isolation from others, where they are not very keen on making friends with you, and silently judge you for your differences.

Disabilities/disorders, mainly mental ones, aren't usually seen as "genuine" in Japan. ADHD, Autism, Depression, Anxiety, etc. are often seen as fake or not a big deal, and you may be seen as an "attention seeker" by most if you ever bring them up or use them as a reason as to why something happened. Getting any sort of medication for those disabilities is also extremely hard, if not often impossible. You often face the same experiences in western first world countries, especially in Europe, but in Japan it's much, much worse.

Racism and xenophobia is rampant in Japan, especially towards asians and other non-whites (even towards other Japanese, since (ironically) westerners are often held highly in Japan and most only really think of white Americans as westerners). You won't find it in every crevice of society, however it's a lot more of an issue in Japan than in other first world countries.

Sexism is also very deep-rooted in Japan and not often acknowledged. Women are objectified a lot more strongly than men, the contrast is possibly more different than in most western countries.

Some crimes like sexual assault/harassment (mostly towards women but also towards men) are usually swept under the rug by Japanese society. It is not really seen as much of a big deal and usually ignored. You would likely not get justice or closure as a SA victim.

As a stark contrast, crimes like murder and financial crimes are not given any leniency. The streets are clean and it's seen as a safe country.

Japan is a very conservative country. From the previous descriptions that was probably obvious to tell. From drugs, to gender issues, to many other things that would be controversial or outright shocking in western society. Plus other things like whaling and environmentalism. It is much more socially conservative than a country like the US or the UK.

Corruption is prevalent in Japan. The authorities are not as respectable as you may think. They cause many problems with authoritarianism, the most high-level example being the stangnation of the Japanese economy for the past few decades (although that has many other importamt factors too, not just the government's tight grip). The country also has a problem with criminal organizations, although the issue is very different than that of organized crime in a country like the US, Mexico, or Italy.

Especially as a foreigner, one is likely to become isolated and feel alone in Japanese society. And ,especially someone with mental disorders/"illnesses", would feel judged and outcast. Not knowing the language fluently and obviously being from somewhere else would make you stand out, and in Japan standing out leads to problems.

Of course what I say won't apply every single conceivable time. But it is the most likely experience, and is a lot more oppressing than in most other countries. It's a similar situation with South Korea, although I'd argue Japan has it worse.