Coming from the old world of business IT where "integrate everything with the relational database" is standard procedure, it's certainly not unusual for the database to be a place where abstractions are defined(1), so what you expose via PostgREST may not in fact be a close match for the underlying data model.

(1 - In fact with me it's pretty much a matter of policy: any external system should access data via views named for the external system. Then, as inconvenient as it sometimes is, Postgres' dependency mechanism will keep me right about which fields in which tables are depended upon by which external systems.)

What kind of resources do you have that can't be modelled as materialized views and make sense to store in a database?