Beware of fake teachers promising results but can't do it themselves.
The best teachers make themselves dispensable. They should help their students be independent learners.
I feel this problem is especially bad at university. If a university professor does a bad job teaching they keep their job or get moved to administration or research.
If a private piano teacher does a bad job they lose students. If a teacher on YouTube does a bad job they lose views.
Students of good teachers can still suffer from quarter in the machine syndrome.
That said, I did have some amazing teachers at university and also some very bad. I claim that the good teachers are good in spite of the system not because of it. The incentive structure of the university doesn't necessarily encourage good teaching.