I like the idea of services like Prismic and Contentful, but yes I find them overkill for smaller projects.

For larger ones I'd be worried about being tied too closely to a company which may or may not be around in a few years - I'd probably look to open source project instead.

For example Drupal has good user and content management features, and with D8 there's more interest in 'headless' Drupal, where it becomes an API backend to custom front-ends.