FHIR is about as far from what I'd call a standard as you can get. Each individual country/region/hospital/department/lab can define "profiles" that configure the data to their specific use case and no-one else's. Not exactly the aim of a standard in the usual computing sense.

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

Does it actually work in practice? I often see 'theoretical' support, you can export data but if you re-import it, it's all fucked.

Like Zigbee, in theory like every company supports it but in practice moat imlementations is not compatiable with each-other

Yes; Epic participates in the CareEverywhere network which has a gateway to the CareQuality network, thus supporting decent interoperability with other hospitals and clinics using a variety of EHRs.

Now, how much they share is potentially open to debate.

Is there any public documentation on Epic and how one can interact with it?

Yes, it works in practice. The trouble is more with the customers than the vendor. Some customers are still using obsolete versions that are missing the latest standards-based open interoperability features. Or they just haven't enabled those features.

My favorite way to describe the issues with FHIR is semantic vs syntactical interoperability.

Using FHIR means syntactically interoperability. Same structure, uses json, things generally in the same place.

Semantic interoperability means that the 2 systems can understand each other… just using FHIR is NOT enough for this. Different value sets and ways of representing the same information mean that 2 systems view of the same data may be very different.

I liken this to when I walked into a bar in a small town in Northern Ireland. I was speaking English, the bartender was speaking English, but we could not understand each other. We eventually figured things out but it was TOUGH to communicate even though we were ostensibly speaking the same language.

This is what FHIR apis from 2 different vendors are like. Same “language” remarkably different dialects

Those standards are the equivalent to JSON or XML.

They still require massive amounts of implementations.

Look at the maturity level of the HL7 resource. Those that are higher (5-6) are going to have more agreement on standards and have more turnkey setup.

For example, condition in the US is likely to use ICD-10. Procedures, especially those billed are going to use CPT or HCPCS.

Epic provides support for their implementation of the standard.

Any person working in healthcare software, which I did for 15 years, knows that for every standard there is a myriad of vendor implementations, bugs, hacks, workarounds, and just saying "System A speaks FHIR and System B does too" does not mean you can just load an EHR dataset from A into B and have all the information usable.