MongoDB being schemaless for me is an easy way to work with the javascript objects in json. I think it would be harder to work with traditional relational databases like PostgreSQL in that approach.
I'm not talking about whatever your specific use case is, I'm only talking about how NodeBB/database works.
If you don't see MongoDB suitable, because its collection model is not used by Nodebb, the table model around PostgreSQL wouldn't be used either with the additional problem of being tied to an specific schema if you don't model all data with key/blob.
What I'm saying is that the relational table structure (SQL) is closer to key/value (Redis) than the document-store structure (Mongo). Obviously that doesn't matter when talking about NodeBB/database since it doesn't using any of those specific features. It essentially just emulates Redis on top of Mongo, which a Postgre adapter would also do.
