Honestly, I get so weirded out when people call on the "Melting Pot" analogy to defend groups having highly distinct sub-cultures as a good thing.
Canada's open about the basic structure, calling it a "cultural mosaic". Enshrining ghettoization as a policy goal, pretty blatant issues arise damned near instantly, because people with incompatible beliefs living face-to-face doesn't work.
Rome's success was the "melting pot" approach of looting everything that worked and assimilating peoples, the Mongols ran a "cultural mosaic" empire because they gave no fucks about the beliefs of those they ruled over.
The key thing to note is that the Muslims ruled a "cultural mosaic" formed of the pre-existing spread of nations left alone. Meanwhile, Canada imports people expecting them to segregate nearly as hard, if not harder, than Jim Crow.
It works for an empire because the different cultures need never have widespread friction, they just share a boss who takes ideas from each as appropriate. It doesn't work inside a city because that friction will be on every social scale possible.
Edit: I'll elaborate on the last line there. "Every social scale possible" is my belief because, to my understanding, you don't really get such cultural friction between discrete settlements, because the bulk of the population doesn't interact.
Unless, of course, there's a Strong Government to leverage with in-group bias. Dearborn is only liable to be a broad problem in the state of Michigan because of having a powerful representative government, as the mandate clusterfuck shows.
If the government is strictly decentralized or held strongly by low-intervention outsiders, then they don't have a club to beat eachother over the head with from beyond routine interaction distance. Such was the Mongol practice.
And yes, this generalizes to justify central governments being either weak or disinterested in the majority. That's why the US is a Federal Republic, we're not
supposed to have California and New York be fucking with "flyover country."
E2: Basically, Cultural Mosaics work if the separate cultures aren't forced to actually live with eachother and lack an external means of oppressing eachother. Modern examples fail both requirements, due to urbanization and government mission creep.