unning an Express instance in a Lambda function, good or bad?

While learning the Serverless Frameowrk I came across several tutorials showing how to run an Express instance in a Lambda. This seems to me like an overkill and against the purpose of Lambda functions.

The approach usually involves running an Express instance in the Lambda and proxying API Gateway requests to the Express router for internal handling.

To me the trivial approach is to just create an API in API Gateway and route individual requests to a Lambda for handling. Am I missing something?

Taking into account that Lambdas’ execution time is 15 minutes, isn’t just spinning up the Express instance quite expensive in terms of memory? Also, limited to 100 concurrent Lambda executions would create a bottleneck, no? Wouldn’t an EC2 instance be a better fit in such case? Using a Lambda like this seems like an overkill.

The only two benefits I see in running an Express instance in a Lambda are:

  1. In the case of migration of an existing app written in Express, allows to slowly break down the app into API Gateway endpoints.
  2. Internal handling of routing rather than relying on the API Gateway request/response model (proxying to Express router).

What would be the benefit of such approach, in case I am missing something?

Some resources promoting this approach: