Craig here from Crunchy Data. Not sure if you mean Crunchy Data is like RDS or isn't, in some cases we're very similar as a managed service provider. But are focused on a better developer experience and quality support.
We've had a number of customers that use us for the database and fly for the app. We had a user benchmark a number of heroku alternatives with various database providers and we were actually better response time than the unmanaged instances on fly themselves in addition to all other providers they tested - https://webstack.dancroak.com/
I won't speak for Fly, but we're big fans of them and think we pair quite well together.