You can achieve exactly the same thing with PostgreSQL tables with two columns (key JSONB PRIMARY KEY, value JSONB), including indices on subfields. With way more other functionality and support options.
Using Postgres with JSON operators isn't that difficult, of course there are some pitfalls and corner cases due to Postgres's architectural choices but then you'll get that with just about any DB choice. And if Postgres's JSON operators aren't to your taste there are JSONPath queries you can use too.
PostgreSQL docs > "JSON Functions and Operators" https://www.postgresql.org/docs/current/functions-json.html
MongoDB can do jsonSchema:
> Document Validator¶ You can use $jsonSchema in a document validator to enforce the specified schema on insert and update operations:
db.createCollection( <collection>, { validator: { $jsonSchema: <schema> } } )
db.runCommand( { collMod: <collection>, validator:{ $jsonSchema: <schema> } } )
Looks like there are at least 2 ways to handle JSONschema with Postgres: https://stackoverflow.com/questions/22228525/json-schema-val... ; neither of which are written in e.g. Rust or Go.
Is there a good way to handle JSON-LD (JSON Linked Data) with Postgres yet?
There are probably 10 comparisons of triple stores with rule inference slash reasoning on data ingress and/or egress.