Comment:

Observations should handle questions and answers of all types. There are questions whose answer is an entity, such as a body site, person, substance, location, medication, etc. While it's perfectly OK to use a code that represents a substance, medication, or body structure, it is impossible to describe those entities more fully by using a reference to a resource. So, you can use a substance code, but not reference(Substance). Wherever a code can be used to describe an entity the full entity should be allowed. Therefore, suggest including ref(Any) as a valid Observation value.

Summary:

Observation.value should allow Reference(Any)