New York City is a poster child for the problems with immigration. It’s full of unassimilated ethnic enclaves. Shared norms and social trust are almost non-existent, replaced with hostility and individualism. Governance is basically impossible. You can’t have grass roots, Tocquevillian democracy in such a fractured society, so instead you have governance by elites and ethnic politics.
I did pick NYC as I mentioned in order to hopefully avoid subjective, simplistic and/or half racist responses. One had to be made.
It wouldn't be necessarily a racist response if made against situations in certain cities in France given the extent of assimilationism of that nation, but I call that comment racist given the critic of NYC. New York through its entire existence has been waves of migrants, multi cultural in its DNA, with shapes and influences that changed over time but some would say is exactly what made this city shine and get so admired internationally: a financial pole, with countless architecture prouesses, exceptional arts, culinary and service excellence.
Those "enclaves" have been there since its genesis. What we see when the model is just acculturation.
The unfounded statement that no country in the world can "assimilate" past a certain ratio of minorities is the viewpoint that societies must reflect one particular fantasy view of some homogeneous ethnical soup.
In that case sure, Japan is heaven, Singapore offers a hell of a picture, and every other societies that adopted yet other models are a myth, politics have little to nothing to do with any resulting conflict, revolt and violence, just some cross culture hate and over 200 borders not firm enough for comfort.
I grew up visiting family there (Queens has the largest Bangladeshi enclave in the country). I lived or worked in Manhattan for a year and a half, and my brother has lived there for more than ten years. The city gives me flashbacks to Dhaka.