The baseline HL7 FHIR standard is more of a toolkit than something intended for real-world interoperability. Fortunately there are multiple FHIR accelerators such as CodeX, Argonaut, Da Vinci, etc which write detailed Implementation Guides for specific use cases. Any organization can follow those IGs and be confident of achieving meaningful interoperability. Some of them are even mandated by federal regulations.

http://www.hl7.org/about/fhir-accelerator/index.cfm