A lot of those points resonated with me.
I started using Jenkins (well.... Hudson) years ago to simplify manual builds and quickly discovered the power it brings. This was really exciting. I am a mobile developer and I also maintain a plugin on Jenkins. All the issues I had to struggle with on mobile platforms are still present on Jenkins. Hard to test, hard to reason (why things need to happen the way they do), poor documentation, lack of examples of "how to do it correctly". I'm pleased to say a lot of that has changed on my day to day work as a mobile develop but I have to say, I really struggle with Jenkins.
One thing that really helps on other platforms (Android / Spring Boot / etc) is having an opinionated way of doing things. Maybe I'm missing something but the bare bones starter project still allows for a lot of deviation meaning plugins can end up looking wildly different.
However, the community is great. It really is. Lots of people willing to help. So I really cannot fault it there. And of course, I and other community members can always update the documentation and contribute back. And we should. But personally I've simply not had time to dedicate to this as I only dabble in Jenkins as a hobby not full time. I need a CI ecosystem that just works and that I can easily contribute to.
I should stress this is my personal opinion and I don't speak for any other user of Jenkins but myself.