Eg, great for learning a technology or programming language, but often not great for advanced features of that language.
Whenever you can, use one of the instructors coupon codes prior to buying a course, otherwise the instructors only get about 27% of the money [1] (or less if it's on mobile due to app stores), which is why I decided to not make a game-programming course a couple of years ago.
[1] https://support.udemy.com/hc/en-us/articles/229605008-Instru...
Decision Making Under Uncertainty https://juliaacademy.com/p/decision-making-under-uncertainty...
Udacity
While courses from Coursera and EdX are also excellent, Udacity consistently makes me work hard and actually expend the energy to understand non-trivial tech.
I worked for 3 years as a Go developer before doing this course and I still learned so much from it. Thank you Bill Kennedy, you're a wonderful instructor!