>> The only valid reasons to choose non-relational databases are scalability and performance.
Agree 200%. And people seem to get a wrong idea of where this argument starts making sense. With the right indexes, I can easily handle 200req/s on my last SaaS running on a $50/m PostgeSQL.
It's also significantly easier to move from a standardized relational model to a document DB than the opposite.
The only valid reasons to choose non-relational databases are scalability and performance.