In this century weather we like it or not the world has become more multicultural and connected but I don't think that this is a good thing because too many different races and cultures in a small space creates tension and some of these immigrants don't assimilate into the Broder culture of the host country. But they bring with them more work which boosts the economy which is a good thing.
I don't think multiculturalism is a bad thing, when it simply means different cultures coexisting within the same society. I think the larger problem is multinationalism in a world where the prevailing view is that every nationality and national group must be sovereign, independent, and free. It also doesn't help that we have nearly 200 sovereign nations already.
Here's a story I saw recently:
The daily struggle to find work for Chinese immigrants living illegally in a borough of New York is a far cry from the picture Donald Trump and other Republicans have sought to paint.
apnews.com
Chinese people living in the U.S. would not be a problem if both China and America were a single country under a single government. But since they're separate countries which could potentially go to war with each other, then we end up with fears that they could be forming some kind of fifth column that could hurt American security interests.
Another issue is multilingualism. Some might be okay with multiculturalism when it comes to things like food, religion, music, art, etc. - but they draw line at language. There are those who emphasize the importance of a common language and oppose any moves towards bilingualism or multilingualism.