The idea that multiculturalism is incompatible with a harmonious society is empirically denied by the existence of basically utopias like Singapore. All citizens have to learn 4 languages and there is a huge amount of tourists which add to the already multicutral demographics of the country. Lowest crime in the world, best education in the world. Among the best public transit in the world, etc.
What it really is is that some minorities have shittier cultures than others, and some states have handled the integration and assimilation better than others.
Even the USA, for example, is blessed that nearly all of our illegals are Catholics (or more recently evangelical protestants...). Compare this to Europe where it's often muslims who try to bring their Burkas and sharia law with them.
The mass rise of latino voters for trump indicates a very strong effort from America's latino minorities to "act white", to "integrate", and to "assimilate".
I am fine with a multicultural society, as long as it's good culture - and yes, it is quite easy to nearly objectively quantify if a culture or cultural practice is good or not. For example, Americans except for asian americans don't wash their asses with bidets after using the toilet. This leads to swampass/BO, increased toilet paper usage/resource consumption, and far more rectal related health problems. America objectively should change it's cultural practices around bathroom usage.
Unfortunately, cultural critique against bad culture has a tendency for folks to call you "racist", so we aren't allowed to have this conversations with more specificity...