You're right, Mobile does not include every content type. It includes the four predominant type: status updates, blog posts, discussions, and documents. Stated another way, custom content types, Videos, Ideas, Polls, Bookmarks, Tags, and Observations are not displayed in activity streams or search. Any links to those content types that are housed within a supported piece of content, e.g. a link to an Idea from within a document, will open a new browser window in an attempt to display that item in the desktop UI.