If you are on AWS, the direct attached storage option you have is ephemeral volumes. That's bad. If the hypervisor fails you lose data. Which is fine, you have replicas. If there's an event that takes out multiple machines at once, even for a moment, you are hosed(this can be AWS issues, or could be as simple as automation misbehaving and shutting machines down).

I'm all for treating DB as cattle, but your cattle needs to be able to survive long enough.

EBS is expensive but it works for PG. AWS uses EBS for RDS databases. Calling that a 'mistake' is a tall order.