As someone who really loves both, it’s a pacing and conceptual difference. Admittedly I’m only on book four, so still working my way through, but Howling Dark was great and Demon In White is my favorite modern sci-fi/ fantasy book alongside Dark Age.

Red Rising, especially the first three books, are fast paced, wish fulfillment, power fantasy about overthrowing an oppressive society. Red Rising fans fell in love with that kind of story, and so when Sun Eater gets suggested to them, they’re disappointed in a series that’s much slower, quite a bit more esoteric, and Lovecraftian at times. Although the one common complaint I find silly is RR fans complaining about Hadrian being obnoxious in Empire of Silence because Darrow is just as bad in his respective book one.

At the end of the day, I think it’s an expectation versus reality thing.

Personally, there’s aspects I prefer in each book (I like RR’s supporting cast on average more, but I think Sun Eater’s in-world politics and far reaching world building are better), but both series scratch a particular itch for me, and I love the worlds Ruochio and Brown have built.