I can't assuage your fears.

I tried it a year or so ago when I was first starting AWS serverless dev. IIRC it supports very few services. Most of our lambdas were behind API gateway and a Cognito authorizer but localstack didn't have a Cognito user pool. We botched something together with nginx to replace CloudFront for a while.

I don't remember quite what the last straw was, but there were big gaps in what we needed. Mind you, I keep finding I'm just one more AWS service away from what I need with the real thing do I'm not sure that game ever ends.

Not sure if you’re suggesting using local stack for production but if you are, I’m not sure that’s it’s intended purpose. This the first time I’ve ever heard of local stack, but just judging by their website, they don’t seem to be trying to replace AWS for production. They are just trying to offer devs a local testing environment.

Absolutely, I'm not sure about the poster's intent, but his concerns still apply to using it for developer testing

If your application uses features that don't work in LocalStack, it can be very tricky to start using LocalStack to test that application. Conversely, if you are already using LocalStack to test/develop your application, that may discourage you from using AWS features which might be a good fit, but aren't supported by LocalStack.

Also, I would certainly recommend against solely testing with LocalStack, since you may run into situations where it behaves slightly differently than AWS, and if you develop targeting LocalStack's behavior, you could end up with bugs when AWS behaves differently in production.

Your second paragraph indeed describes my question:

If I invest in a AWS-but-not-AWS dev tool, will it be good enough?

Terraform has an escape hatch, local-exec, which u can use to call aws CLI directly.