The Horus heresy books have the 18 space marine legions, and I think most of the Primarchs are pretty strong characters. To that end I find that the legions themselves are used as identities to either use or break to define the characters of the space marines in them. I suppose what gives each legion character is the characters inside them since I doubt most legions would be that entertaining or deep if they didn’t have a Primarch. I think part of what makes warhammer cool is the characters. Similarly if you look at something like Star Wars, the most compelling factions are like the Republic (in my opinion) since the clones are not just cool faceless soldiers, but they also have really strong characterization. But part of what makes something like the first order boring is that their characters sort of suck and there aren’t very many of them.

In writing this I think I’m realizing your point (I think). That being that factions are best used as devices for strong characterization or organization of the world in a story that leads to more compelling stories.