MongoDB is a document store which has much better querying and update capabilities than PostgreSQL's JSONB/HStore. It's also probably an order of magnitude faster when it comes to updates.

Not sure why you brought up Cassandra it is not a document store and completely irrelevant to this discussion.

Postgres used to benchmark faster than MongoDB as a document store when compared to MongoDB running with a reasonable reliability setting. Not sure if that has changed in the last few years though - Mongo has improved a bit, and Postgres is getting faster on each release.

Let the tired old irrelevant Mongo-flamewars begin!

As much as I agree, the arguments are a bit tired, I think the criticism can be useful.

There's a tendency in web development towards using well known stacks, and some of them are quite poor choices for most projects. MongoDB features heavily in these stacks despite relational databases being a better fit most of the time, possibly because there's less visibility of alternatives in the JS community.

Do you think the OP is storing documents? I will put cash money they are doing something like the MEAN stack and storing relational or semi relational data in Mongo. Not many startups spend the majority of their storage storing documents.