> Everywhere that implemented them had their scientific, economic, and artistic progress completely stunted

I think you're dismissing a bit too easily soviet russia. Under communist command, russia rised to the status of superpower.

As far as I know, khmers rouges were "reactionary" marxists : for instance they refused western medicine and dubbed peasants as "old men" and city folks as "new men". This is not what marxism is about.