I would argue against that. As a whole relative to most other societies, I do feel the US is about as close to a multicultural society as you can get. Sure, prejudice, racism, and inequity due to cultural or ethnic background will exist to some extent in any society, but in terms of general acceptance and treatment towards people of different culture/ethnicity and economic and social opportunities, I can think of few places where minorities have an easier time. Asian Americans in particular have it better than many other demographic groups within the US, and the average and median income/wealth of Asian Americans are the highest out of any group within the US.
Tolerance is not assimilation, and the last fifty years have been a time of abundance and prosperity for the US. How people act in times of plenty is no judge of how they will act in hard times.
We don't even need to go outside of the US for examples of how minorities are treated when **** hits the fan. Japanese internment, Muslim surveillance, Chinese exclusion, the list goes on. The usual counter argument that this all happened before the Civil Rights laws of the 1970s is hilarious because contrary to all the propaganda regarding rule of law,
nothing in the US is sacred. Just look at how Trump stomped all over civil liberties, is trying to eliminate jus sanguinis, and remains the most popular Republican candidate for the next president. The fact of the matter is that all that's required to change US laws and even the Constitution, is public paranoia.
Hell, it's happening even now, as academics of Chinese heritage are being witch hunted throughout universities. When, in the next great war, Chinese Americans are thrown into camps, don't say we didn't warn them.