Europeans from small countries love to tout their welfare policies as "so much better" than places like the U.S., but they ignore one massive difference: Small European countries are localized, ethnically homogenous populations. The U.S. has to deal with more land, and more people, from more places. You cannot make an apples-to-apples comparison between welfare systems in the U.S. and small countries.
https://www.cia.gov/library/publications/the-world-factbook/... ~80% White/Dutch
So both nations have a similar level of "ethnically homogenous populations".
No.
There is more than "white" and "all other ethnicities." Considering geneology is important when designing systems like healthcare, for example. If a disease is isolated to a gene present in only people of Irish descent, and not Scandanavian descent, then the country with more Irish people will pay a higher healthcare cost than one with less. The more unique subsets of genes present in a population, the greater the ethnic diversity. Since the U.S. is comprised almost entirely of immigrants, it will naturally have a more ethnically heterogenous population than a country with population centers dating back millenia.