Use Postgres, problem solved.

While I like relational data and I _love_ Postgres specifically, there are some tasks and contexts for which non-relational stores are better suited.

I'm sure that there are tasks for which non-relational stores might be better suited, but pretty much anything MongoDB does, Postgres does it better and in a more performant way (esp after the optimizations to the JSONB data type in the last few major releases).

This is a very bold comment, explain how you scale write, do HA in pg?