Based on my experience, apart from a handful (really a handful) of open courses by elite universities (e.g. Harvard's Stats 110), the vast majority of online courses are garbage - low quality of content, insufficient coverage of the material, inappropriate format, bad teaching. I'm talking about almost anything on Coursera/Udacity/Udemy. Even if the courses were good, you still need to spend a good amount of time with a textbook and practice the subject. And considering that most courses are useless, you're usually better off skipping them altogether and getting straight to the book + practice approach to learning.