I actually meant about them being bad author insertion characters. Just read Battle of the Fang, for instance. Magnus flies back and forth from being all-powerful to having to "remember what he was" and it's just a tad silly. Like they want to keep them in the books cause they're popular, but they don't really know what else to do with them if there aren't other Primarchs around to fight them. They seem to stomp anything anyone else anyone throws at them (Avatars of Khaine, for instance,) and are only really defeated by their own dumb mistakes and poor judgement.
Like the C'tan, they're cool characters that don't really have a place in 40k combat. I'm ok with that, I just wish they didn't ignore all the rumors and hints they'd set up prior to this codex. All those dozens of threads over the years about the Void Dragon being on Mars and not a single mention of it in this codex. There was in the first codex, however, which is why I believe it has now been retconned into some other type of entity.
EA: The pattern has repeated itself more times than you can fathom. Game companies rise, evolve, advance, and at the apex of their glory, they are extinguished. Bioware was not the first. By utilizing our funding, game companies develop along the paths we desire. They exist because we allow it, and will end because we demand it.