[znep]

This is very true, the costs and performance impacts can be significant if your architecture isn't designed to account for it. And sometimes even if it is.

In addition, unless you can cleanly survive an AZ going down, which can take a bunch more work in some cases, then being multi-AZ can actually reduce your availability by giving more things to fail.

AZs are a powerful tool but are not a no-brainer for applications at scale that are not designed for them, it is literally spreading your workload across multiple nearby data centers with a bit (or a lot) more tooling and services to help than if you were doing it in your own data centers.