I think it's just the trade off between these two scenarios.

- Relatively poor amortized scale out time with good guarantees in the worst case.

- Good amortized scale out time with dropped requests / timeouts in the worst case.

With lambda, it doesn't really matter how spiky the traffic is. Users will see the cold start latency, albeit more often. With Fargate, users won't run into the cold start latencies - until they do, and the whole request may timeout waiting for that new server to spin up.

At least that seems to be the case to me. I have personally never ran a docker image in fargate, but I'd be surprised if it could spin up, initialize and serve a request in two seconds.