At the moment the userver framework is in the "beta" state. That state is meant to show that the framework is in state of migration from in-house to a fully open source development:
Moreover, as there's no release, there's no API stability yet.
We are successfully using userver for ages for our in-house development for many high-load high-availability services.
Make an experiment to make sure that the current functionality suits your needs. If there's something missing, please fill a feature request at github.