Lore/rules dissonance reminds me when in Rogue Trader, SM was a force comprised of human, violent criminals with maybe cybernetics here and there, basically the French Foreign Legion... IN SPAAACE as far as I understood. Certainly they were weak as kittens compared to current SM.
I don't think even lore is that accurate, IG brass would come up with MEQ as in the number of IG grunts required to entertain a single Khornate berserker for a minute or so where he'll stay still and not pay attention to the AF jocks nuking him from the orbit, if SM were living up to the hype.
Same problem happened in Bungie's Myth series: first had Trow which were the fastest units in the game who'd require at least a dozen of berserkers to put down, and even then most of them would be punted into the orbit like it was an even more grimdark Blood Bowl. First time Trow appear there's only one and it's enough to put the fear of God into you.
Second game Trow are on your side, so designers had to up the tension somewhat. Behold... Trow, but he throws exploding skulls at you!
Third game some genius came up with the idea "wouldn't it be cool and scary if Trow had actual armor and weapons?" and it is until you realize that the missions featuring them are fucking retarded and borderline unplayable, so you go for (cheddar) cheese tactics like running circles around them like you're baiting a retarded puppy while your archers plink them to death between naps and smoko breaks.
I don't care much about Star Wars but I find it funny how EU tries to explain stupid shit from the series. Why stormtroopers can't hit the broad side of the barn and wear eye-searingly white combat suits in a forest environment? Certainly not because they're kids movies made to sell plastic garbage.
Ergo: it's either lore accurate or not fun to play.
BONUS:
If primarchs looks like this:
then to be truly worthy of your Spiritual Liege you have to have the psychique of this: