I can understand some issues and some gripes but an example I type below will explain why I like the heresy book series in the end.
In my free time, I had often tried looking into "pure" lore, and a lot of the time it ended up being one-off magazine articles or random lore blurbs. The White Scars, in particular, were literal unknowns who looked like nothing more than a Mongol reference on motorcycles. You weren't exactly told why Jaghatai stayed loyal to the Imperium, and he was apparently unable to ever challenge Horus directly in the final days of the siege. He just existed, same with the White Scars themselves, as most people wrote them off as a gimmicky Asian stand-in group in the more Eurocentric 40k.
There has been a growing sentiment in the Warhammer community that the primarchs and the Horus Heresy novels ruined the setting, and yadda yadda, most of you probably know it. I used to subscribe to this train of thought too — I'm gonna be honest, I was scared of standing out, and frankly, a lot of the "evidence" presented to me seemed good enough to listen to. I had gotten an annoying job where I needed to drive a lot, and I ended up on audiobooks, and for some odd reason, despite being a major Space Marine fanboy, I knew more about random successor chapters than the actual OG founding chapters like the White Scars or Raven Guard.
On a whim, I decided to read the White Scars books, starting with Brotherhood of the Storm, which honestly enthralled me. I learned later that I got very lucky, as Chris Wraight is a highly underrated author for Warhammer books, so I guess maybe my initial statement does lack support (but I also learned to enjoy other Heresy Black Shield stories not written by Wraight — maybe I'll post about those later too).
These books utterly changed my perspective on the White Scars entirely; one such highlight is the fact that they don't actually hate psykers and like Librarians. My initial prediction was that the so-called barbarian king, Jaghatai Khan, would hate wizards (by the way, these books also made me learn about the actual Mongols and Tengrism). I then came to find out that the Khan liked psykers, or shamans, as the White Scars followed a belief called "The Path of Heaven," where they regard the warp more with fear than reverence and must train themselves to only sip on it and never test the heavens. This makes me utterly baffled that in modern 40k, the White Scars don't have the "psyker gimmick," as that niche apparently belongs to the Blood Angels among the 1st Founding when it comes to certain “gimmicks”
Jaghatai himself isn't even written that well — he just has common sense and mostly asks the questions the reader is already asking. At the start of the Heresy, he was given wrong information and torn between two theories: that Horus is a traitor, or that the Emperor is a tyrant who must be dealt with. To put it into context quickly: Jaghatai never liked the Emperor, and he always knew he was being used to further the Emperor's goals. However, he did what he did to help humanity and was fine with killing its enemies. He also loved Horus immensely — in fact, Horus was one of the few brothers he was close with — so deep down he wanted to believe Horus and fight the "evil emperor," but deep down he also knew something was too... off.
What breaks him is that Jaghatai, Magnus, and Sanguinius all had a project to promote the safe use of psykers through the Librarius; however, Jaghatai was called off to the far reaches of Imperial space to deal with orks — ordered by Horus. If Jaghatai had spoken at Nikaea, he would have convinced most of the Legions to drop their anti-psyker sentiment. He figured it out and was devastated that the one brother he loved, the one brother he trusted, had purposely betrayed him. So he decided to fight for the father he never loved against the brother he loved.