I feel like this encompasses every incarnation of 40k. Online discussion, TT, video games. everything
*Pokes his head in* ...This is still a thing? Well you know, objectively, the best Primarch was Horus, it's why the Emperor elected him Warmaster, ultimately he was the biggest failure but the entire reason it's a tragedy is because Horus was the best and brightest. Even when he talks about Sangy being the Warmaster instead, it's clear that he's having a severe moment of self-doubt rather then any factual evidence. That's what the lore states anyway, you can't show that so well in the Horus Heresy though because...well...if you just make someone the best then they are a one dimensional mary sue, but at the same time characterizing him brings in subjectivity. It's all a tangled mess that Games Workshop weaves.
But you must take into consideration that this was a time of all out war.during th scouring guilliman didn't have time to worry about religions sprouting up. I don't necisarily agree with the codex but I understand why he did it, he didn't want one man to control such large forces.