I think that Cloud Atlas is awkward as an example of the representation problem, because unlike most, it had an unusually diverse cast to begin with. So I suspect that they were not being stingy with roles out of bigotry, oversight, or laziness. There is a controversial argument to be made, I think, that the Cloud Atlas approach is more progressive than most are ready for, demonstrating that racial/ethnic distinctions do exist, but are not as significant or immutable as some might assume.
Besides the Euro/Australian Hugo Weaving as the presumably Korean Boardman Mephi, they also played two English and an American, which nobody seemed to find controversial. The Korean actress Doona Bae played also Tilda Ewing (Celtic?) and an unnamed Mexican woman. Chinese actress Xun Zhou plays a Korean clone, an American, and a Euro/Hawaiian. The Euro/African actress Halle Berry played a Pacific islands native, an Indian partygoer, Luisa Rey (American), Jocasta Ayrs (English), as well as Ovid (Korean?) who has even more drastic makeup than Weaving:
The main reason why Cloud Atlas gets viewed as problematic IMO is because it begs comparison to other productions where “transracial” casting is done to avoid diversity, rather than to celebrate it.