Teams often assume the hard part of connecting to an existing EHR is the transport standard. In practice HL7 v2 and FHIR are both well documented and widely supported, and the mapping work is finite.

The difficulty is identity. Two systems that each hold a valid record for the same person, with different identifiers and slightly different spellings, will happily create a duplicate unless you decide in advance how a match is confirmed.

Agree the matching rule before writing any integration code, and log every automatic merge so a human can reverse it. That single decision determines whether the integration is trusted in six months.