I want to make a small correction - this is not true:
iirc I had to reboot every time for it to be applied while with Arch you can just install something and run it immediately.
nixos-rebuild behaves like most package managers: it makes new packages available immediately, and restarts relevant systemd services. Like other distros you have to reboot to run a new kernel.
And cleaning up Steam games is as issue with most distros too. But I kinda see your point.
Btw Nix (both NixOS and the Nix package manager running in other distros) has this feature where you can run packages without “installing” them if you just want to run them once:
That puts you in a shell with one or more packages temporarily installed. The packages are downloaded to /nix/store/ as usual, but will get garbage-collected sometime after you close the shell.