Nix forces you to serialize every build step (what it calls a "derivation"), and moreover it isolates the build environment to only include things built with Nix or verified by hash. So while there is a lot of power, the only thing you can do with that power is produce derivations which themselves actually run the build.
Contrast this with Gradle, which is currently digging itself out of a hole by forcing authors to declare all inputs and outputs of their tasks so it can serialize them, but you can literally do anything Java can throughout the entire process. This is the kind of Herculean task which is neatly sidestepped by tightly controlling the DSL environment (inputs/outputs) as does Nix.
Contrast this with Gradle, which is currently digging itself out of a hole by forcing authors to declare all inputs and outputs of their tasks so it can serialize them, but you can literally do anything Java can throughout the entire process. This is the kind of Herculean task which is neatly sidestepped by tightly controlling the DSL environment (inputs/outputs) as does Nix.