Epicor REST comes in two versions (v1 and v2) and each of these are available in two flavors: OData and Custom. The custom interface is just a happy wrapper around the business objects you know and love. The OData is a, well, OData compliant interface to those same BOs (YMMV).
As Utah explained, using Functions (which are like BPMs in many ways) encapsulates the Epicor logic away from the client making upgrades much easier. You get to define your own API.
The next big thing would be Webhooks. Epicor could publish events (on_new_part, on_new_order, on_changed_user, etc.) and subscribers would be notified of the event. Subscribing to other webhooks can be done with an ASP.NET application which then talks to Epicor. Maybe this can be done with CDC, but I have not tried it yet.