Boy in 2020 I can't see describing databases as difficult to scale. Expensive, sure, but these are solved problems, and the tradeoffs for going the NoSQL route will likely exceed its perceived benefits.
Modern relational databases are great at what they do.
They are until they're not, at which point it's too late. I deal with things teetering on the edge.
At some point you get to machine sizes that won't fit in AWS as an example.
That's true of, well, anything. A non-RDS / NoSQL datastore doesn't save you here!
I am out of touch of what is available in the NoSQL side of things but about 5 years ago there were too many things which had a terrible failure mode somewhere in the stack thus I'm never sure which way to go instead.