Nix bombs out when it runs into a cyclic reference, so at least infinite recursion doesn't hang without any output. Likewise, it also rejects multiple definitions for a value (though to be fair some values like attribute sets are mergeable and it can be hard to wrap one's head around that).

The service model in Guix looks pretty neat! I've long been thinking it would be nice to have a configuration system for NixOS that uses isolated components with a defined (type-safe) interface. The current modules are kind of that, except it all gets squashed down into a single nested attribute set, so it's not really isolated and (understandably) can be confusing. How does Guix System handle multiple services competing for the same global resource (port allocation, file in /etc, ...)?