Everything in this chapter so far assumed the decision had been made. It usually has not. The M in MERN is a convention, not a requirement, and the honest comparison is narrower than either camp's marketing suggests: PostgreSQL 1,289 is not "the relational one that cannot do JSON," and MongoDB 1,815 is not "the fast one."
The useful question is which of your constraints is hardest to move. If your data has one shape and correctness across several tables at once is what keeps you awake, a relational engine gives you that for free. If it arrives in varying shapes and changes structure every few months, a document store lets you ship without a migration on every deploy.
