When adding a wikilink, Visual Editor provides a dialog with autocomplete for local main namespace articles. There is no autocomplete or easy way to add link to an article from another wiki. The contributor is only able to do this if they know the relevant interwiki prefix ('w', 'meta', 'commons') and already know the target page name.
VisualEditor also does not currently know of custom templates, like this one, which some wikis use for wikilinking.
It would be nice to have this all fixed.
Example interface from one of the comments: