>>96402212
Marines, as the flagship faction, are kind of boring and one-dimensional by necessity. This in and of itself isn't a dealbreaker; it's something with wide appeal to draw people in and, ideally, they'll "graduate" to playing a more interesting army. Where it goes wrong is in the wider setting where, despite taking place in a vast galaxy full of peril and war, the game is basically "Humans vs. Evil Humans" and the nonhuman factions were underdeveloped and generally unimportant, and this turns marines from being "poster boys" to "obvious writers' pet."

Fantasy allowed the non human factions to actually do shit and be important players in the story. Two of the longest ongoing conflicts (Elf civil war & Dwarfs vs. Greenskins vs. Skaven) all involve nonhumans. Hell, there's a separate human faction that arguably co-shares centre stage with The Empire (Imagine THAT in 40k, lol)

Also Primaris Marines and returning primarchs are the lamest goddamn thing since watching paint dry. Mary Sues, created by a Mary Sue writers' pet, undermining the whole point of The Imperium (A dying, bloated, corrupt empire in it's final days fighting desperately against the dying of the light) since, "Oh, look, turns out the situation isn't as hopeless as it looked and we can just bullshit out a thousand more chapters of bigger batmen and all the loyalist primarchs were hiding in a fridge somewhere."

TL:DR Horus Heresy was a mistake. So was the End Times, for that matter.