I like a good deal of what's on Udemy, but it feels like for reasons totally different from the marketing. For me it's more like the online version of those video lecture series you used to buy on VHS or DVD through a "distance learning" mailorder catalog, and not so much like an online classroom. There's some course-like content in it beyond the lectures (quizzes and such), but imo the main value is really just the lectures, and the quality of the lectures is what differentiates good courses on there from bad ones.
Which is fine, video lectures are a model that's worked for a few decades, but now online and more convenient to shop for. And a good video-lecture course is certainly worth at least $10. I guess I can see why they don't brand it that way (the "disrupting education" hype train has money/press, while "mail-order video lectures, but on the internet" is a harder sell). But for me, their courses made a lot more sense to me once I started thinking of it as a place to buy a video lecture course, like the way you might order DVDs of the Feynman physics lectures.