Recs.
Updated
Specs
Pros
Pro The ability to create and edit files on the web UI gives GitHub pages the same power as a small CMS
One of GitHub's features is a very powerful web editor which helps users edit or even create files right from the web browser, once the file is saved it's the same as a commit. Coupled with pages, this tool becomes even more powerful, giving users a free CMS that is easy to use and create.
Cons
Con Unable to set cache expiry, must accept GitHub defaults (which are short)
Low cache expires - GitHub sets the cache-control: max-age header to 600 seconds, or ten minutes. Normally, you would set this value to a year so that it stays cached, and then use fingerprinting on your assets. Instead of serving style.css, you would serve something like style-62c887ea7cf54e743ecf3ce6c62a4ed6.css. As it stands now, assets are rarely going to be cached on repeat visits.
This will give a low score on https://developers.google.com/speed/pagespeed/insights
with a 'should fix' recommendation around 'Leverage browser caching'.
For a high traffic site this may have implications.
Con Doesn't make sense for private websites
Github pages are made for hosting public facing website. Private use websites should be hosted on a different platform.
Recommendations
Comments
Out of Date Pros + Cons
Con For a custom domain users still have to use "www"
Due to GitHub's DDoS mitigation technology, users who host their static websites on their servers, cannot have a custom root domain. For example: "example.com" cannot be used, "www.example.com" must be used instead. While in theory the first one is allowed, it greatly affects loading times.