Nixy has been updated, transforming from a framework into a utility function that allows users to use it with other frameworks while ensuring core functionality remains intact.
Nixy provides an additional evaluation layer. It automatically scans and collects paths, injects option values, and only allows the modules used to flow into the final configuration. This greatly simplifies configuration and improves flexibility while maintaining native compatibility.
Welcome to the new version. 
example configurations - GitHub - SaltedFishesNG/nixos
I like the look of Nixy because it would encourage clean design of your cluster configuration.
So the Nixy staged eval approach provides conditional modules (traits) with high-level options (schema) to encapsulate the nodes’ nixos configurations.
I imagine there are deploy-time performance benefits too, right? The deployment tool would not need to evaluate all nixos nodes in the cluster together.
Possibly, yes. nixy avoids evaluating all nodes together. Traits that are not enabled will not be included in the final configuration.