It's hard to find a use case that a plain old filesystem can't handle at small to medium scale. But there are perhaps more important considerations than just "can it handle it"
Uh let's not get carried away. It's fine with enough work maybe. But Postgres has a lot of awkwardness too. HA is a pain, major version upgrades are a pain, JS or JVM stored procs are a pain, configuring auth is a pain. There is a reason so many people are desperate to pay someone else to run Postgres for them instead of just renting a few VMs and doing it themselves.
It's the best, but it's far from perfect. Default mode is non-ACID, and going `serializable` mode makes it very slow. Spanner is always ACID... but always slow.
