I'll probably get downvoted. But isn't that exactly what make does? specify how an output file should be made, and which input files it depends on.
Nix provides the toolchain and dependency management (like where boost is from), as well as ability to remotely execute it which makes it attractive to scale out large builds like a `-j 999`. Nix also lets you do things like patching boost and then recompiling both boost and the downstreams (incrementally and with early cut-off if using nix-ninja) all in one build graph.
All in all, probably not useful if you're not already needing features from Nix. But if you are, this should speed up your builds by a significant amount.
Less features is probably good, because make originally was just the dependency thing, but got abused as a build system.
Any performance improvements such as incremental compilations / incremental builds / interpreters / copy-on-write / hot-code reloading are always welcome.