Well, for starters there's the risk of cultural diffusion mixing up a whole bunch of different cultures and ending up with some nasty melting pot-type deal like we've got in the United States.
There's an inherent division in society that wouldn't be present in a culturally/racially homogenous one.
There's oppression, which finds its way into pretty much every multicultural, diverse society.
That's just to name a few, there are more which I'm too lazy to list right now.


