Firewatch and The Witness are still pretty low-poly. Unturned isn’t “blocky” charm, it’s the low-poly (and flatter texture) charm. The Witness does it really well, without overly complex textures.
I don’t think it’s about a substantial difference in polygons, but really how stuff is being designed. The Witness looks really nice because of how the lighting interacts with objects, creating a more natural feeling for the boulders and trees with more concave or jutting parts.
It’s ambiance as a whole that’s important, which leads into hopefully beautiful soundscapes and sharp audio clips for actions. Clean animations. Nice effects for lakes, waterfalls, and rain, and a honed-on mood for it all.
Currently, there are mixed reviews for the ambiance of Unturned 3. Mostly, this is about how it doesn’t feel dreary enough to people, especially with the (albeit curated) Hawaii map. It more-or-less pretty closely follows the art style of Unturned overall, but it loses its charm to people because there is less blending of the ambiance as a whole. That in itself is an entirely different issue/concept to think about. (Ambiance as a bigger priority?)
(Although Hawaii has other reasons as to why some people don’t like the map, this is one of the more notable ones people read about.)
tldr: lots of words to kinda explain my thoughts on 2/3’s of the second sentence and then this for the last 1/3 not actually covered above: I haven’t seen a zombie game in the art style of The Witness or Firewatch, and now I really want to. Most games go for that higher-poly “realistic” style.
Also take this image. It highlights a lot of that relatively well, but ofc an “updated” Unturned art style would not be 1:1 to either game. I’d imagine it’d be a bit lower-poly than this still, especially so on some of the smaller angles on the rocks. But those rocks also look far nicer imo with some of the additional crevices and distinct separations.
Kinda hard to imagine it “dreary” though. Two very different games, so it’d be two very different takes on this style.