The skepticism is founded, but there's some truth.
PostgreSQL (and traditional RDBMSes) aren't built with horizontal scaling in mind. That's both a blessing and a curse. The curse is obvious. The blessing is that it means that they will let you get much further with vertical scaling. Especially if you take the time to learn how to tune them.
Support for horizontal scaling comes at a cost. When I've done performance analyses, I regularly find that parallel and distributed implementations spend most their wall clock time on synchronization and communication. Occasionally more than 90%. That creates a big up-front cost you need to overcome before you see a net benefit. It also leads to a sort of horizontal scaling version of the rocket equation where returns can diminish rapidly.
PostgreSQL (and traditional RDBMSes) aren't built with horizontal scaling in mind. That's both a blessing and a curse. The curse is obvious. The blessing is that it means that they will let you get much further with vertical scaling. Especially if you take the time to learn how to tune them.
Support for horizontal scaling comes at a cost. When I've done performance analyses, I regularly find that parallel and distributed implementations spend most their wall clock time on synchronization and communication. Occasionally more than 90%. That creates a big up-front cost you need to overcome before you see a net benefit. It also leads to a sort of horizontal scaling version of the rocket equation where returns can diminish rapidly.