I'm pretty sure the answer is usually "concurrency". The examples alluded to in TFA sound like it. Handling concurrency is notably extremely hard, and an RDBMS

I'm pretty sure the answer is usually "concurrency". The examples alluded to in TFA sound like it. Handling concurrency is notably extremely hard, and an RDBMS is what other applications use to solve their hairy concurrency problems so they don't have to.