I lived for one year in Cambodia. Major roads are mapped, so you can generally navigate, but lots of small streets and roads are missing. Also, even in the capital most of the buildings are not mapped. I've mapped a few buildings and streets but didn't have time to do a lot. Also, it's incredibly hard for a foreigner that doesn't know Khmer language.

Mongolia is poorly mapped on all maps, not just OSM. But OSM coverage was the poorest about one year ago. Don't know if it is improved since then.