I ran into the maintenance load of an SSG for my blog, and only just now switched themes over this rather than fixing the old theme (which had several customizations). In that theme swap, I think I lost all the productivity I gained from using the SSG over raw HTML.
One other productivity gain though is that if you end up switching SSG engines entirely, you still have your source files. Those could easily work with the next one, or at least leverage trying others out. If everything is baked into rendered HTML, it will be much more work.
