They also tend to become interop nightmares due to having an incomplete/ambiguous/non-existent grammar. Different implementations end up implementing slightly different rules which ends up requiring an accumulation of hacks to parse all the variations that end up in the wild.
This is the HL7 spec's issue. Everyone interprets the spec slightly differently. It's given rise to the interface engine, which are a type of very powerful software that sits between systems and makes things work properly, which is why I love them.
