Currently, Pyro will hard-fail if the key HKLM\SOFTWARE\Wow6432Node\Bethesda Softworks\Skyrim Special Edition (or similar for other games i guess?) is missing, or set to a non-existent directory.
Thing is, that key is not required at all, as long as --compiler-path is passed. Setting the key to any arbitary folder will still work, as long as it exists, which feels kinda pointless.