Fascists are grossly incompetent, and that's one of the things that's missing in almost all "satirical" portrayal of fascists, which makes sense since Western society was only ever against fascism during WWII when they were forced to be. And it's very telling how it was during 1939-1945 when satirical portrayal of fascists actually highlights how stupid and incompetent they are. Part of it was just how popular slapstick was during that time, but seriously, go watch actual satirical portrays of fascists during that time. The tone is completely different from modern "satirical" portrayals.
There's the classic Schichlegruber Doing the Lambeth Walk, which took footage from Triumph of the Will and edited the footage so it looked like Nazis were dancing to some popular music that the Nazis themselves denounced as "Jewish mischief and animalistic hopping." The obvious message of the short film was that Nazis were incompetent buffoons who only knew how to march and not even march that well.
The stupid and incompetent WH40k faction are orks, not space marines. Space marines are what fascists think they are while orks are what fascists actually are. Hell, even calling them orks gives them too much credit because orks aren't that cowardly.