As much as China has a dominant ethnicity, this is only really true in the eastern, core parts of China. In the Northern and Western fringes, plus Tibet, there's entirely unrelated groups of people who, at times, want nothing to do with Beijing.

There are countries with more ethnic cohesion, though they tend to be smaller: Iceland for example. Any larger country either came about by annexing smaller ones, or was sufficiently valuable enough that it was colonized at some point.