How much of your API is dependant on GIT like semantics and how much could work with "plain files" storage?
It's neat to be able to transplant a CMS on top of other services that don't necessarily deploy by CI pipes, I made a DAV plugin(basic auth) for Netlify (back before the forks) so as long as a server supported listing(only DAV dependency really), GET, PUT and DELETE you could just plunk down the CMS files somewhere and point to the DAV share.
I believe Netlify CMS has an actual backend interfacing with the various Git systems they supported. Pages CMS is different in that it's a pure front-end app. I have a couple tiny serverless functions to help authenticate you with GitHub, but then after that there is no backend involved. That means I need some API for the front-end to interact with, at least for now.
It's neat to be able to transplant a CMS on top of other services that don't necessarily deploy by CI pipes, I made a DAV plugin(basic auth) for Netlify (back before the forks) so as long as a server supported listing(only DAV dependency really), GET, PUT and DELETE you could just plunk down the CMS files somewhere and point to the DAV share.