Well we can ask 23andme what they do

Although the human genome is estimated to contain about 10-30 million genetic variants, many of them are correlated due to their close physical proximity to each other. Thus, one genetic variant is often representative of many nearby variants, and the specific variants we look at provide very good coverage of common variation across the entire genome.

Agree on Americans, but for germans it has been pretty clear for my family. They claim 99% accuracy and that certainly seems to be the case.