Yes, I do know teachers from such countries that are excellent teachers. Their knowledge base is solid. South Korea, from what I have read, is now requiring certified English teachers to teach there. The origin of the teacher matters less now as more Filipinos are going there, plus other nationalities.

The basic problem, though, is being able to understand what the teacher is saying. I have met African and Indian teachers that have such heavy accents that I don't know what language they are speaking. A teacher might be a good teacher, but intelligibility might be a problem.

From my experience students who learn from teachers with heavy accents are only trained to understand that accent. Those students who learn from teachers with a clear accent have better listening skills and speak better. All my middle school students put an "uh" at the end of their words because all their teachers speak the same way. I have five year-old students who speak more clearly than my middle school student because they are learning from me. The five year-olds can also speak in sentences.

The best E.S.L. teacher I ever met was a Hungarian woman who sounded like she was born and raised in the U.S. state of Iowa.