a. Using nix in place of a package manager means dealing with libraries specific to that language. It's still managing different apis. And more potential for bugs and unforeseen problems in custom 3rd party APIs as opposed to the official one. Admit it, you hit tons of problems getting everything to work fine with python.

b. C++ is the only one that would benefit from nix here because C++ dependencies are usually installed externally. There's no folder with all the external sources in the project. Even so this can be achieved with docker. If you want you can have docker call some other scripting language to install everything if you want "one language" which is essentially what you're doing with nix.