AWS provided recently a library [1] for running chaos experiments [2], which look like an evolution of the work their own employee did before [3]. I wonder if the FIS is just a re-packaging and proper integration of these or something more.
Chaos Runner, or anything else that you run on your EC2 instances (or in your ECS containers) requires you to install some kind of agent or daemon. This is non-optimal as it is work in and of itself, it is also hard to correctly simulate things like the network, or various AWS services being down/unreachable.
If you run the agent/daemon on your production stack, then it's a potential vector for misconfiguration or attack. But if you don't run the agent/daemon in production, then it's another way in which your test stack diverges from production!
I saw various PR/FAQs related to Chaos engineering while I worked in both EC2 and the AWS developer tools org. I've been gone over a year now, but I would bet that FIS does something at the EC2 Network level so that you don't have to install stuff on your instances or containers.
[1] https://github.com/amzn/awsssmchaosrunner
[2] https://aws.amazon.com/blogs/opensource/building-resilient-s...
[3] https://github.com/adhorn/chaos-ssm-documents