Since 0.7, you can pass a db option to LevelUP to swap out the default LevelDOWN with anything LevelDOWN-API compliant, allowing you to seamlessly (save an npm install) store your data in all of these.
When creating a multilingual forum, I faced the problem of distributing content in different languages.
I liked how this problem was solved in invision power board.
To create a multilingual forum, I need to develop a plugin that disables unnecessary languages and adds additional fields to the platform settings for adding content in the required languages. To begin with, I need to have additional fields for the names and descriptions of categories, as is done in invision power board. Who can do it, send your suggestions in the chat
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.