munin_update relies on a stats file that exists, but isn't found in the default location on NixOS; the appropriate plugin configuration is added. munin_stats relies on munin-cron writing a logfile, which the NixOS build of munin does not. (This is probably fixable in the munin package, but I don't have time to dig into that right now.)
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.