>"that "Asian" lumps together too many really distinct groups"
So does white/caucasian, and probably all the other race "colors" as well. I.e. They lump together many different geographically-isolated/defined peoples and cultures into one singular entity for simplicity. And then we're led to believe statistics that are somehow indicative of something regarding that "race".
Hmong households have slightly lower median incomes than the total US population, for instance.[0]
[0] http://www.hndinc.org/cmsAdmin/uploads/dlc/HND-Census-Report...