One scenario where I find external wikis better is when a change encompasses multiple components and you want to present an overarching view to the user + want to refer across sections. This can be managed via something like github pages for each component but the cross referring across multiple repos adds more friction. Plus to present the overall view, something like a synthetic "doc-repo" will be needed that has landing pages that connect the component-level pages. Or we can have a wiki that does the integration by refering to component level github pages but at that point why not just do the whole thing in wiki much more conveniently.

My experience is that it basically comes down to developer discipline. A disciplined developer will update the docs regardless of whether the docs are part of repo or external and vice versa.