The artbook in question, くらやみの世界 by circle Ox/ツクモイスオ, is no exception. As one can easily see from their Pixiv profile, Ox's work pretty much approaches professional quality, possessing that clean, shiny, digitally enhanced polish that is pretty much the professional standard in Japanese illustration (and you can also buy the dōjinshi I'm reviewing on their Booth).
くらやみの世界 , a fairly slim (around 20 pages) artbook from 2016, is split in half between landscape concept art, and character art. Visually and technically, there is very little criticism that can be leveled against Ox's work: they nail the perspective and volumes to a pinpoint, the color palettes are varied (perhaps sometimes a tad bit too chaotic), and the large, two page spread format really makes the artist's fantasy cityscapes justice.
The contents themselves are, however, a bit too... by the numbers. I mean, there's nothing explicitly wrong with Ox's designs; yet, they play really close to the tropes of that generic J-fantasy that, personally, I am not a big fan of. Let's look at the character designs: there's the little dark witch, the human prince, the anthro, the robot... even the landscapes, while well constructed an executed, could thematically come out of whatever SAO clones happened to be the flavor of the week in 2016.
All in all, a very nice display of skill, but I can't help wishing that the artist had pushed the envelope a bit more when it came to the... imagination part. This is really a criticism I could move against most Japanese fantasy illustration today, so I hope Ox doesn't take it personally...