[0] https://www.nytimes.com/2017/11/30/world/asia/japan-lonely-d...
https://en.m.wikipedia.org/wiki/Burakumin
Caste-based discrimination was an important civil rights issue for most of the 20th century, and still has long standing effects on the organization and opportunities for people in Japanese society.
And of course there’s also Zainichi Koreans, long standing Chinese communities in places like Kanagawa, the urban rural divide, and countless more divisions that counter you’re simplistic description of Japan as a uniform society.
Despite there being many poor, segregated communities in the USA, crime is perpetrated by and happens in only a few select communities. In fact, many poor communities thrive because of segregation. In an integrated society, members of the poor communities would be marginalized due to language and cultural barriers because there would not be a support network of people from similar cultural backgrounds. (although the communities where crime is commonplace would benefit from integration by spreading the crime around to different communities instead of keeping it within the community)
Japanese culture and values across all groups are much more homogenous and conducive to a civilized society. People look down upon crimes and value hard work and education.
On the other hand, there are communities in the USA who have values that are not conducive to civilized society and that glorify crime and hedonism and look down on the educated. Coincidentally, these communities also disproportionately contain the perpetrators of crimes.
On the surface level, it appears that the value system of the disruptive groups are the problem because many segregated, poor groups can live in civilized peace. Segregation itself is not a problem.
Everything points to the malfunctioning culture and value system of specific groups in the USA. Integration may help in so much as it dilutes the toxicity of the culture and value system of these groups.
The key to having such a degree of integration is to spread the good equally between social groups - there should not be first and second class. Everyone must be in the same class. Otherwise, the discriminated class will resent it and think of themselves as not part of the same society, so, why respect its rules? Example: black neighborhoods in NY.
The Japanese way of reducing crime - by reducing class difference and keeping everyone well connected in the social web. But I perceive the risk of reduced diversity, lack of creativity and innovation in that kind of system. Integration must be balanced by differentiation and vice-versa, if it is to work well.