Youtube videos are fairly good starting point for learning mixing and mastering and music production as well! Yet they are still are a sort of self education way solution. Cheap, versatile, convenient and easy available... And still one faces question constantly buzzing in head - why the hell these guys doing that and not opposite?
No explanations available at all!
Yet time taking classes are time saving. Yeah, it takes some funds but teacher's experience who's able to fix bugs and lead right way can't be underestimated.
So to me classes are still the best solution wor those who aims pro skills at shortest time!