I read Red Rising a while back and can't remember if it was that or the sequel made me think "Braveheart" meets xxx - and I can't remember what the xxx was. I hadn't realised it was supposed to be YA. Boy has YA got grim and violent. Just because the protaganists are teenage doesn't mean it's right for teenagers - bit like saying Lord of the Flies is suitable for pre-teens. I gave up on probably the second book - and I only read them from the library, didn't buy.
I think some of the popularity is it is like a highly coloured Holywood movie, everything a bit larger than life with large, obvious dramatics. I wouldn't call the characters well developed. They were all a bit one note to me - it's all about the game.
I did note the author is I think a psychologist and the books smacked to me of having been based on a lot of theory of group interaction - only interacted with group psychology a little, in context of work training courses, and the odd documentary on why genocide/civil war happens but it just had a "feel" of that to me.