For us the ECS autoscaling feature is way too naive. We need to do our own controller on some more responsive metrics. The customization support is pretty bad.
Would you mind saying more about what you had to build, and what you would liked to see? One thing I'd like to see in the future is scaling actions that supported multipliers, as well as metric math support (scale service to request_count/acceptable_requests_per_service).
One thing we experienced at Segment was the fact that we needed to quickly handle a surge in volume but couldn't overload partners. Essentially we wanted something that scaled up quickly at first but was pretty conservative after that.
We settled on constant increases/decreases using queue depth thresholds but ideally ECS would support feeding multiple metrics and doing some basic math to figure out how much we're currently process, how long it would take to flush the queue, and how many more containers we need to drain in a timely fashion.