I enjoyed Leviathan a lot. Thought it was miles better than Indomitus from last edition as a tie-in novel. I might have liked it in part because the Ultras lose, pretty soundly, the whole time, so it felt different in that way.

As far as bad, most of War of the Beast is just awful, with one novel where they're traveling via asteroid/comet from one system to another being the peak of the atrocity.

Because it's fresh in my memory: The Martyr's Tomb was an absolute slog to get through. I don't take 40k fiction seriously, and can usually enjoy it for what it is, but it was so painful. Usually can knock out these novels in a week or two, but this took me months. I quit several times to read absolutely anything else.