Our former Art Director, Ghislain Barbe, had this to say on the subject:
"There are probably two pieces that inspired us the most and those were: the first Black Library's Soul Drinker Omnibus cover by Kai Lim, who was kind enough to do some illustrations for us afterward, and the Dark Angels Codex by Raymond Swanland, because it was the first marketing illustration that was ever linked to our game, even before I got on board, so it's the first thing I saw related to the game. Both pictures are simply fantastic. They have a lot of movement, they are dark and gritty, they have dramatic lighting, and a lot of material quality in their rendering.
For the characters models, we were determined not to get creative with the IP but to stay as close as possible to what people see when they imagine their miniature in real life, so we really based ourselves on them for all the details, all the iconic elements, but we had to adapt the shapes and proportions from the heroic scale to a more realistic one, and also so that they animate well and look cool. A lot of decisions are often made for purely technical reasons.
It is difficult to say that we are realistic when we do characters that are transhumans such as the Adeptus Astartes, or aliens, like the other races, since we lack a proper human reference for comparison, so there definitely is a level of stylisation whether we want it or not, but we tried to stay as close as possible to what we imagined they would look like in real life. That's for the geometry. For materials - textures and how lights react on them - we also aimed for realism - contemporary realism, that is, because realism in CG is always linked to what technology is available at the moment - but we tried to go a step beyond realism, so that lights affect more dramatically the characters than they would do in real life, so that it's a bit too much. But then again, we're dealing with ceramite, armaplast and such, which are materials we know nothing about in the 3rd millennium, so what we think is stylisation might just be it."