I've been running Fly in beta for one of my apps and it's pretty damn cool. Adding a couple more today.
Our site is a good example: the static docs are hosted on Github Pages, the landing page is a middleware + Rack app, and our dashboard is a Rails app hosted on Kubernetes. All of these are mounted under https://fly.io. The edges are aware of the session cookies Rails uses and we have routing rules to send people to the right backends if they're logged in.
It's early, but the middleware is already super powerful.
It doesn't say service discovery (it should!). This will mostly let us build some neat features, though. We can let people build rules to send traffic to a particular branch.