I think there's very little "proper" usage for EFS. That is to say: if you had infinite time and dev resources, you'd pick something else because other options

I think there's very little "proper" usage for EFS. That is to say: if you had infinite time and dev resources, you'd pick something else because other options are faster and cheaper. But it is very easy to set up EFS and start using it since it's just a filesystem. The time it takes to migrate from "my code deals with local files" to "my code deals with networked files" with EFS is about 15 minutes regardless of what you're doing with the files. S3 can't say that. I think EFS's value proposition is more about that convenience than it is about actually being an appropriate tech choice when considered in a vacuum.