NixOS likely only refused to run it because you weren't running it in the Nix way. That's not a jab or anything, Nix has a huge learning curve and requires doing a lot differently. You're supposed to use devshells whenever doing development. If you want something to just work, you use a container.

Whatever issue you ran into most likely had nothing to do with NixOS being immutable, and was probably caused by the non standard filesystem hierarchy, which prevents random dynamically linked binaries from running.

I've never heard of flatpak and immutability being obstacles to developers, in fact I generally hear the opposite. Bluefin is primarily targeted at developers, and some apps, like Bottles, will only officially support the flatpak distribution because of the simplicity and benefits it brings over standard distro packaging.