I feel like any problem that Postgres can't handle is a good problem to have. Either you've got so many customers that you're hitting sharp edges, or you're working on such an interesting problem that you're out of the domain where Postgres is helpful. That I should be so lucky

A moderate-size queue is already a problem that Postgres struggles to handle %) But fortunately such problems are rare, and relatively well-known, like, say, doubly-linked lists in Rust.

For most other problems, Postgres works well, or at least well enough.