Even without using reserved instance pricing, ECS on EC2 is much cheaper, isn't it? At work we use Hasura, which is written in Haskell and cannot be (easily?) run as a Lambda. Our alternative solution is to run it as a container on ECS. Given that it's a permanently running service, with Fargate we'd pay just to have it sit idle for half of the time, and Fargate is not cheap.

Even when running non-reserved EC2 instances to make up our ECS cluster, it is cheaper than using Fargate.