Say whatever you want about modern web development, but I think React/JSX + HTML/CSS has proven you can use a declarative model to build complex user interfaces.
Take on the other hand, some of SwiftUI's more advanced features, like PreferenceKey which are extremely under-documented (the generic sounding name doesn't help either). Of course folks are going to struggle to do more complex things.
I wonder how much the situation would be helped if the thing you searched for help allowed in-line SwiftUI examples, like you can do on the web. Playgrounds is kind of that thing but you aren't searching a repository of playgrounds for documentation.
Watch this talk from one of the core developers about the design https://www.youtube.com/watch?v=dkyY9WCGMi0
In fact, Dart doesn't even support closed source.