Welcome to the Prismic community, and thanks for reaching out to us.
This is true that fetchLinks can not retrieve the following fields:
Link to Media
Rich Text (anything other than the first element)
Any field in a Group or Slice
That's why Prismic implemented graphQuery (an advanced version of fetchLinks or fetchLinks V2) that enables you to make advanced queries. You can do selective fetching (to retrieve only specific fields) and deep fetching (to retrieve content from linked documents). That's the solution to retrieve linked documents.