Works just fine here. There was a time when there was something wrong with the graphics stack and it crashed on start, but it runs just fine now. NixOS 24.11, launching it from getty with

    environment.loginShellInit = ''
      if [ "$(tty)" == /dev/tty1 -a "$USER" != "root" ]; then
        exec systemd-cat -t niri niri --session
      fi
    '';