If you're just launching a standard app, pretty easy. However, anything complex requires writing a ton of AWS SDK code. (For example, we launch a spot instance with a given task definition, assign a given customer's work to it, then shut it down when the work is finished, and then on to the next customer)

Too complex, KISS principle rulez :) Creating ECS Cluster on spot instances seems like a way to go. Haven't tried myself yet, in to-do's :)

We had a simple setup before (and have used spot instances everywhere it made sense). Without getting into the specifics of the use case, the ability to target a specific instance for specific backend processing tasks became necessary. Rest assured, it's not "too complex", only "as complex as necessary". Be careful not to respond with a rote "you're doing it wrong"-ism when discussing technical merits.

Can you elaborate? Can you give an example of a specific problem you ran into with ECS, and how you were able to easily solve it with K8S?

I couldn't get my nginx container to communicate with my app container in ECS however it all worked locally. And theres very little docs or troubleshooting resources.