All of the Horus Heresy books were mistakes. The Black Library stable was utterly unprepared to write 30k as a meaningfully different setting from 40k, and it was a problem that exploded the same way the book series did. And it changed 40k in general for the worse as well, as all those writers then started porting their Heresy-era stuff in 40k to write sequel books and consequences which which shoved all of 40k's other aspects off to the side to focus entirely on the Chaos/Imperium war, AKA the stupidest conflict that can't ever be resolved without blowing up the entire setting a la the old world, and the conflict which makes the Imperium retroactively justified in literally everything because the need for escalation kept giving Chaos more and more power and because Chaos is such a darker-than-black faction all the Imperium's grimdark nonsense morphed into po-faced 'but we have to' as justification to keep Chaos at bay.
Like once upon a time, M41 was supposed to be a shit time to live in even by the standards of 40k. When we saw the crumbling infrastructure, the deadlocked government, the ineffectual justice system (huh this is all sounding really familiar...) we were supposed to be seeing a galactic civilization in its last days before the big collapse. M40, M39, M38, etc, they were not like this. They had bad times, but in M41 the bad times had swelled to encompass all times, and the bad times got even worse. But then were started writing M31, and it turned out to be just a slightly different flavor of grimdark even pre-Heresy, and M41 turned out to be nothing terribly special. And then we started porting the primarchs into modern day because primarchs are cool and also GW needs to release a new line of space marines that they can ferociously copywrite because they're butthurt about being slapped down in court.
15 years of the HH series gave 40k brainrot, and that's before we get into the absolute nonsense in the series itself like the constant retcons, the navelgazing, the idiotic shock twists, the grimdarkness of the far past, and especially the perpetuals and the assassination of MLK. Were there a few good ideas? Were there a few good moments? Were there a few good books? Yes, but they got drowned out by the sheer amount of awful coming out of BL and GW for the better part of ten years. It's only recently as the Heresy series winds down and people seem to be waking up to the fact they really can't milk it forever that we're starting to see more alien-centric stuff again, and I can't wait to see the Imperium/Chaos stuff to take a god damn backseat for awhile.
Now before someone else points it out, I'll do it myself - all the problems 40k has with its fanbase and its darker-than-dark thematics existed before the Horus Heresy series. But the Heresy series made it worse by bringing a laser focus on the worst aspects of the franchise and also trying to be Mature and Thoughtful and Philosophical which ended up carrying so much water for the worst elements of their fanbase while GW sat quietly and raked in the cash and only recently started speaking up once it became clear that their brand was not just attracting literal Nazis (who have always infested the 40k club) but that their IP was becoming more and more widely recognized as being associated with Nazis.