Yeah, it's unfortunate that the work is so collaborative at times. Having read all the published Horus Heresy novels to date, where by now each Primarch has been written by at least three different people depending on the story they show up in, tone and characterization can change pretty dramatically. To the point I swear some writers have said Sanguinius has blond hair, while others write he has raven black hair. You could I suppose chalk that up to the classic "mythical details" part of 40k, but if the HH is an attempt to show things as they really were, the least the authors could do is agree on WTF color their hair is. It's also hard to keep the character of the Primarchs in mind when they've been passed through so many hands. Only really the broadest generalities stay the same, but language choice, how sly or humorous a primarch is, how angry....I mean, I'm reading one right now where Angron forms cogent sentences and calls people brother and laughs....versus the first few HH novels where he's literally a beast and can barely talk without "ugggnnnnn......rrrrrrrrr........gggrrrraaaaa....." inserted between each actual word, and crushing some World Eater skulls.
I'd like to think their personalities are so big, so prolific, that they are discernible regardless of who is writing them. But that turns out you end up with all the primarchs acting and sounding the same.
Maybe I'll make a fun exercise, go crawling through my HH for Primach quotes, remove the names and ask people to guess who said it.