That can be true if you are using tightly coupled event sources (like s3, SNS, etc) where you need to inspect the incoming object. If you are doing a HTTP / REST API, try to decouple the code as much as possible from the API Gateway invocation by using ExpressJS / aws-serverless-express to aid in local testing and debugging. Then testing and debugging locally becomes much easier.
This is my preferred style, and since an express app is technically just a function that accepts req/res, you can pass a whole express app in as the function when running in the cloud, so you have multiple path routes, middleware, the whole nine. You can actually host a fairly complex API in a single function subject to the package size/dependency count restrictions of your FaaS of choice
