From the courses currently offered on the major course sites it seems they limit themselves to courses taught during the first two years of college - and many courses are dumbed-down to a "general public" level. There are a few exceptions, for example the quantum mechanics courses on edX, but they really are exceptions.
You can get more demanding courses on the university learning websites themselves, for example https://lagunita.stanford.edu/ for Stanford.
The breadth of the courses and course-takers is focused on the basics though. That doesn't mean it's all easy and dumbed-down - a course like "Medical Neuroscience" on Coursera (excellent, by the way, the professor is a wonderful teacher - https://www.coursera.org/learn/medical-neuroscience) have a workload of >15 hours/week. It still is "only" an introductory course though.
I think you can sure get the basics via online education, and that means "university level basics". The main problem I see with more advanced courses is that you need to test your level of understanding and have it challenged on a high level, way more than is good for an online course. That's because those tests, if they are any good, is going to show you the huge gaping holes you still have in your knowledge, and for an online course that's too discouraging.
The usual way here during education is to get a worksheet parallelly to the lecture every week, work through the problems and hand them in for correction and grading.
If it's easy, the solution and correction are straight-forward. But if it's hard, as, say, some mathematical proof, sometimes a lot of knowledge and work goes into determining the correctness of the solution. And these are the problems that I feel changed the structure of my mind over the years.
And then, even if things are easy to determine right or wrong, as for a coding assignment, people still profit immensely from getting good feedback on their work, from qualified people (and not peer-grading like online courses occasionally do).