hmm, what do you mean and to which platform?
I was thinking of enterprise users and companies who mostly start with a cheap cms, then add features here and there then an app, then an intranet thingy and at the end you have a spaghetti architecture, because everything has it's own front-end without a unifying API. It's rare that developers write their own solutions, you can say, hey nobody would do that, but the reality is far from that. Most devs simply use what's there and try to integrate different projects "by their own way".
I found some things that can be done better at prismic.io and think there is some wasted potential there.
- It requires writing your own controller that needs to implement caching or you have a 500ms delay for API requests
- No multilanguage from the beginning (gotta bake your own solution, that's not so fun)
- Slugs for objects in collections are ugly (see blog example)
Here are all REST API based "CMS", unfortunately, none of them is actually perfect. http://www.programmableweb.com/apitag/CMS?protocol=REST