Practices on EPIC are still fundamentally non-networked, so even with an "API", you can't scale your product from one practice to all of them without some amount of non-trivial integration.

I work at athena and we're dogfooding our API for an increasing number of products/components as it's an easy way to both decouple internal services and improve the API for our partners. From my lowly, uninformed position as a developer, we're growing the number of partners as fast as our team can support at this point.