One of the great things about stuff like Convox and Empire is that, because they're built on existing stable technologies and utilize AWS managed services, you're basically asking if ELB, EC2 and ECS can scale. The most unstable component in all of this is probably Docker. In our experience running it in production at scale, it's faults have been pretty minor (things like push/pull being agonizingly slow) and failure modes get taken care of by ECS managing desired state well.

Yes, we have people using this in production, including ourselves! Most of the scale right now is on the small-to-moderate end but once your application is up and running you're really only limited by how much AWS you want to pay for :)