Uriel Ventris has ptsd now, but he still loves his holy Codex Astartes.
Warriors of Ultramar is a pretty standard novel. It doesn't really stand out in any particular fields, the plot details are somewhat interesting and the action is done well enough, but everything else is mediocre to poor.
The character Snowdog and his crew of other bizarre named characters aren't especially interesting and take up a decent enough chunk of the book. Not much can be said about any of the guardsmen, other than that they are guardsmen. As stated previously, Urial loves his Codex Astartes, and those Mortifactors sure don't love it as much.
Overall, the book isn't offensively bad, its just a fine novel, which may be enough for anyone who particularly liked Nightbringer.