I'm sorry but please do not believe anything this commenter is saying.
I've been a "managed partner" with Udemy for 5 years now (600k enrollments) and I have never once been asked to sign a contract. Despite my repeated attempts to get them to promote my courses, they do not do anything special for this secret cabal of instructors. I still launch courses on Udemy that crash and burn spectacularly. No, they do not manually rank your courses for you. Your course ranks for its keywords and following a fairly straightforward system that considers student satisfaction, keyword density, and search list CTR (and other things).
Udemy does have problems and chief among them is the "disgruntled instructor problem", as in there are a lot of instructors who did not find traction on their system and are convinced they were somehow screwed and it was all a conspiracy to steal their content. It's the same dynamic on YouTube. For every 1 Youtuber who makes it big, there are 50 that don't manage to crack the code and find success. Most platforms are like this.
If your course didn't do well on Udemy, there are generally 3 reasons: 1) your content isn't as good as you think it is 2) you made something that is quantitatively worse than an existing course on the same exact subject. 3) your topic doesn't have demand or your offer just isn't compelling. It's really that simple.
9 times out of 10 when you hear the "uDemYY is A SC@m" screed, you can know they fit one of those 3 reasons. Ask for a link to their course. It's almost always disorganized, poorly produced, on a topic that's already saturated, and with confusing copywriting.
Figuring out what works on a platform takes time and iteration. It's fairly rare that you hit it big on your first shot. Some people push through it and eventually find their stride, and some people just don't (and often blame the platform for not recognizing their genius). It's a MARKET. Their algorithm ranks based on quality metrics. If your stuff doesn't rise to the top, then your course doesn't have what the system wants.
Per the "build your own audience" argument: Yes, you do not get access to the direct email addresses of the students. This is a tradeoff you make to work with them. They can drive you 10s of thousands of enrollments a month, but it is after all their system and so they're going to control what they can. The beauty of making a Udemy course is you don't have to DO any marketing if you make content worth buying. Personally, I enjoy making courses but find selling them to be a massive headache.