In my experience, everything on Udemy is very introductory, but the courses I have taken have been great. As with most online learning, the responsibility falls on you to stick with it and make sure you understand the instructor before moving forward.

I dont think I would trust the "nanodegrees" that udacity is giving out, but I've also heard people using the knowledge they learn from the nanodegrees as a more lateral move in careers to a similar field rather than learning an entirely new topic.