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.