How good is Yandex indexing and search technology?...My guess is that probably not even close to Google, and heavily optimized for slavic inflections and cyrilic.
I am not aware of any single South Korean who has used either Yandex or Baidu. They don't even support Korean at all, so why would any Korean bother to use them? Very interested in the source of your statistics, which I guess completely incorrect.
I heard that nowadays search engine quality is largely determined by traffic that's used to infer relevance from user behavior. Less traffic - less quality. Bing, for example, has decent engineers, but since Google is already ahead it will stay ahead if only just for that reason. So I was told.
So Yandex doesn't have that much hope outside of Russia. Unless a dramatic breakthrough is made, of course.