>

While those of Latin Nations can assimilate more easily thanks to our shared Christian / colonial history, I believe America is assimilation phenomenon has much more to do thanks to our birthright citizenship policy. The moment the child is born here, their status as a citizen is no longer in question. In other countries without birthright citizenship, their identities and stature in society comes into question due to lack of citizenship, despite living on that country for the same length as their citizen peers. This leads to a dichotomy of identification, which often leads to second and third generation migrants (whome should be citizens by American standards) doubling down in the roots that their parents originated from.
That, and the entire structure of the EU makes assimilation very difficult, since the elevating of different cultures is a core aspect of national sovereignty. The preservation of identities and languages, extends to non-eu people, which in turn makes it more difficult for people to assimilate to the hosting states. Whereas in the us, there's sort of a sense of "duel identity", where you can be whatever background you'd like, but you're also American on top of that. Which makes integration easier, since legal status is more straightforward and untied to generational background.
Globalism is retracting. It didn't so much fail as it has just run its course.
We will see a great age of protectionism and regional alliances. Countries self reliance will become important.
Immigration in a lot of EU countries has failed. They have failed to correctly assimilate the immigrants into their society. Say what you want about american culture but something about coming to america makes you american..... >

The quality of immigrants also makes a big difference in addition to the tolerance of the host countries.
The US attracts immigrants with a higher educational and socioeconomic status. Furthermore, those immigrants are themselves attracted to the American dream and are motivated to integrate and succede. Indians or Asians for example have achieved higher average income status. 
The only sore spot is the southern border which gets vilified even though it provides lots of cheap labor for basic industries.
Europe is kinda the opposite where their bulk of immigrants are not really interested in their culture or lifestyle and are just looking to make a living after being forced to leave their host country.
The guy from Afganistan doesn't give a shit about Swedish values. He probably thinks they are a bunch of degenerates, but really likes their free social welfare so he feels resentful and stuck