If it's decoupled, it's plenty testable.

Serverless covers 2 scenarios very well: when you have volume so low, that you don't want to pay for reserved capacity: if nobody is using the stuff, you pay next to nothing.

And also if there is a lot of load, it's very very easy to scale it to cover very high demand.

SAM provides a pretty painless way to test locally and is core to most CI/CD with Lambda.

Unless you use custom API Gateway authorizers, want to interact with other AWS services or ...

Don't get me wrong, I absolutely love AWS SAM, but I believe testing serverless applications locally is still an unsolved problem.