I think Gav Thorpe and A D-B ‘’get’’ Astartes, in how they act and think. Same for Abnett. The rest of the Black Library authors get it to varying degrees. Of course some of this matters on different Chapters.

For example, Graham McNeill does not write very good Ultramarines at all, portraying them as a bunch of hidebound idiots, a massive disservice to the chapter IMO. A D-B’s short descriptions of the Marines Errant was a better description of a Codex Chapter than McNeill did in six novels. William King’s Space Wolves are vastly superior to Lee Lightner’s and A D-B’s Flesh Tearers are IMO superior to James Swallow’s depiction of them.