Nix and its companion NixOS are fantastic. Cross-compiling toolchains are the kind of thing that you have always been able to do, it would just take you two days of fiddling to set up.
With Nix, one person can do that work and it can work perfectly first try for anybody else with just a git clone and a command.
In my APrinter firmware project[1], I rely exclusively on Nix for building for multiple microcontrollers, including AVR and many ARM ones. It's great - I can tell anyone "just get Nix (on Linux) and run this command". They can even run the configurator as a local web application, all that is needed is nix-build then run it.
I even took it one step further and made a nixops deployment expression for setting up an online service (like http://www.aprinter.eu/), which will itself compile firmware for you as configured.
In fairness, though Nix does take the pain out of cross-compilation and other operations through a higher-level package and configuration management layer, a saner build system and toolchain would have accomplished the same in the particular case of cross-compiling: http://aiju.de/rant/cross-compiling
With Nix, one person can do that work and it can work perfectly first try for anybody else with just a git clone and a command.