Designing applications for the right amount of scale is a huge architectural task. With serverless part of it is handled for you, but some of it you're on your own.

Serverless already has high availability out of the box. Do you need to add on top of that?

When deciding to go serverless, you need to put up some guardrails to protect yourself. If you don't, you might find yourself in a hole you can't get out of.