It'd be like WP: an absolute nightmare. On a project I worked on (and sadly, I haven't had time to fix up, though might take some time out to do that, actually), I had a WP blog which I had two categories in - Article of the Week and Announcements, and I wanted to pull that into the homepage, which was outside the WP installation.
Were I doing it in SMF+SSI, it would be 10 minutes work total for the two items I wanted. It took me 2+ hours to figure out the structure, correctly, that WP uses then to write the appropriate queries.
