The "problem" is that a database as a queue doesn't infinitely scale, but nothing infinitely scales. If you're starting to reach the limits of Postgres-as-a-queue then you've either done something very wrong or very right to get to that point.

It's probably more correct to say that the engineering effort required to make a Postgres-as-a-queue scale horizontally is a lot more than the engineering effort required to make a dedicated queueing service scale horizontally. The trade-off is that you're typically going to have to start scaling horizontally much sooner with your dedicated queuing service than with a Postgres database.

The argument for Postgres-as-a-queue is that you might be able to get to market much quicker, which can be significantly more important than how well you can scale down the track.