Well, in the world of free software you have a choice - an easily-scaled database system that offers you few guarantees and is very hard to code safely against, or one that is harder to scale but easier to code safely against. That doesn't make PG bad - it's just a choice you make. If you want to pay, you can get a little bit closer to having both.

Of course you hear lines like you quoted all the time - that's because it's true. The overwhelming majority of applications don't have needs beyond a master-slave pair of DB systems - and trends are moving in favour of that direction every day as RAM gets cheaper. Stack Overflow runs on a single not-that-beefy master-slave DB system.

Typical web applications are insanely amenable to read caches, and that's where you do most of your scaling. If you're at the point where your scaling needs truly exceed a large single DB system, I'd hope you have some money to throw at the problem.