In one of those odd moments in which a politician accidentally speaks the truth, German Chancellor Angela
Merkel said in 2010 that multiculturalism had failed. She said that the idea of various cultures living in harmony, "side-by-side" didn't work, that it had "utterly failed." She's since changed her tune, of course, but she wasn't wrong.
Multiculturalism does not, indeed cannot, create a peaceful and happy socio-cultural climate. By its very nature it is about division and disunity, about separating people of various cultures and encouraging them to stay separate from the rest of society.