Let me say first that, I care most about data integrity, monitoring and maintainability, rather than performance; and I do have relational data.
There was a talk, and pretty much consensus amongst experienced and vocal people, that PostGRES is preferable to MongoDB.
However, I see this as not necessarily between RDBMS and NoSQL, but as the SQL syntax, and availability of services.
I don't need that many rows right now, BTW.
Open source databases are actually free to host, but hosting servers are usually not free; not to mention maintenance costs and security. (How far do I need to know DevSecOps?)
Furthermore, how much are research in embedded database currently being done? Are we pretty much stuck with SQLite? Or, are there better ones for which languages? (maybe, Neo4j or H2?)