I think I have seen a Github issue on raml2html for improving rendering of union types. You might want to look at it and probably give your feedback on what you would expect.
Rendering union types is not the easiest and there are different things to consider. Feedback and help to understand options would be immensely helpful to the team.
The API Console from MuleSoft does display union types, but also lack a more deep navigation which you might want.
I know different teams are working on improving the rendering experience.