I think something the Warhammer community on Tumblr needs to remember...
Is this section of the Fandom is a lot smaller than the vast majority of people into Warhammer. Namely the hopeless plastic addicts like myself who throw a frankly ruinous amount of money at GW to add to the pile of Shame.
So, when we on Tumblr ask why the Primarchs seem controversial in the wider Fandom space, its worth considering the giant hive city that is the Tabletop Fandom.
The Primarchs are controversial because they're the biggest symptom of Games Workshop screwing over every faction that isn't the Imperium or Chaos for a decade and a half.
Ninth Edition was meant to be the Necrons' time to shine. The Silent King came back. And we're only THIS YEAR getting a novel about what he's been up to. Five years since he came back. Five years of nothing for a character who has had a bigger impact on the backstory of the setting than the Emperor and the Primarchs and its not even close.
The Warp being as fucked up as it is, including the existence of the Chaos Gods? Szarekh was partially responsible for that.
The Orks existing? Szarekh and the Necrontyr and the C'tan's war with the Old Ones are the reason for that.
The Aeldari becoming a galaxy spanning Empire and eventually creating Slaanesh? Only happened because Szarekh ordering the Necrons to sleep left the Galaxy open for them.
The C'tan being shattered, the gods broken? Szarekh.
The Pylon network that Abbadon spent 13 Black Crusades trying to break? Szarekh.
The Necrons waking up again? SZAREKH because he found the Tyranids in Dark Space.
He is probably the single most impactful figure in the lore and how many novels has he had since coming back? One. In five years.
What was the biggest, most hyped model release for Ninth? a Primarch coming back.
The Primarchs had most of the Siege of Terra in that time.
(This is just the Necrons. The T'au created their own warp god and she seems to be one of the few universally benevolent gods in the setting. She's never appeared again. The Ynnari had their plotline taken out back and shot in the head to make room for Fulgrim's return. The Orks great WAAAGH under Ghaz hasn't got off the ground.)
This, of course, isn't to say its wrong to like the Primarchs! I love the Primarchs. I love the Imperium. But I also fully understand why so many people, especially Xenos fans, don't.