I started doing Linux tutorials on YouTube a few years ago ( http://youtube.com/c/tutorialinux ) and then created a course on Udemy about a year ago. Affiliates make a decent percentage there (75%) but I suspect they have the same frustration that I do: Udemy has sales EVERY FEW DAYS.
Every few days there are sales that offer 90% off all courses, $10 courses, etc. etc. So a course with a $200 price tag will have an average sale price of $10-15, which leaves everyone (including affiliates) with pretty slim margins.
The simple problem is that they have the power to re-price courses as they see fit, and an incentive to do so (Udemy gets 50% of the revenue, possibly more from their promotions).
As it is, I make about the same amount of money from YouTube revenue (40k subscribers) as I do with one of the highest-rated Linux courses on Udemy (a few hundred dollars a month). It's really just a blip next to my salary; I can't see it replacing full-time tech work anytime soon.