I think viewing this in terms of society-level power dynamics is a little too simplistic, and we can show that by example.

I have a white friend who moved into the Mission District in SF, mostly sight unseen, and hated it. For those not familiar, the Mission is a primarily hispanic community with lots of loud music, partying, street races, and people setting off fireworks at night. My friend from out of town was miserable with all the noise and had to break his lease early, he just couldn't understand how people could value their communal celebrations more than people's peace and quiet. Conversely, my friends who who grew up in the love it and being immersed in that culture is a huge source of their happiness.

While I'm not sure I'd use the phrasing "multiculturalism doesn't work", I think there's a real phenomenon here where people with like values are happier self-segregating into communities of people with similar values rather than integrating more fully and compromising on more of the things that are important to them.

> Ehh. I think it sometimes sucks for those for which the past lack of equity benefitted them.

Absence of multiculturalism is not inherently a "lack of equity", and dismissing any criticism with "you just liked it because you got to oppress people" is, well, dismissive. This excuse might work if you are only familiar with the US perspective, but living in a largely "monocultural" country myself, I can attest that it is not benefiting from "lack of equity" that makes it wonderful to live in. How could it, when, even assuming there is a substantial lack of equity, minorities are so statistically negligible that their oppression cannot significantly benefit the majority.