The problem with multiculturalism is that it has not been shown to actually, well, work.
Assimilation works. Comes at a price, but it works.
Cultural hybridity works. Doesn't necessarily come without cost, as the cognitive dissonance of being one way with one group and a different culture with another is mentally straining.
True multiculturalism has not worked past the individual level.
Culture is an aspect of identity and identity is a human need.
On the other hand, you are also correct in that truly homogenous nations have had as many and as challenging -- if different -- issues to confront as have more heterogeneous societies.
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