If I go to a random wiki and type a username in the search field, and that user migrated his user page to a global user page on Meta-wiki, I'll not be able to go directly to this page on the local wiki, because there will be no suggestion in the list.
As an example, I tried this on https://es.wikipedia.org, where my global user page is loaded (so, links to it appears in blue color, including the one in the personal portlet). The only item which appeared was "containing... user:he7d3r", but there should be a bold entry linking directly to it.