If you did well with the Imperial Guard, you’re going to find the other three factions an easy romp. My experience playing Imperial Guard after Space Marines and Necrons was OMG these guys are crazy fragile and do no damage to anything. I can’t believe my units are getting torn apart by a single unit of Kroot Hounds. That was followed by “what? my guard unit has just been taken over by an enemy unit and now is under someone else’s control”. I’m not entirely sure why that never happened with my Space Marines or Necrons, other than their morale is a bit higher.
I didn’t get that far in my little experimental Imperial Guard campaign. But from what I saw, Imperial Guard is playing a different, much harder game.