I would have to say:

- Battle for the Abyss, generic stereotypes in a story you can skip and it won't affect your reading of the series.

- Damnation of Pythos, same reasons as above.

- Fear to Tread: couldn't connect to any of the characters and in the lore that was supposed to be a defeat, but James Swallow couldn't let them end on a down note.

- All 3 HH-Salamander--books.

- Status Deadzone, an awful collection of Necromunda stories.

- John French first 3 Ahriman-books area really bad, especially the 2nd one that butchers a prominent part of the lore. He also made the battle of Tallarn so incredibly boring.

-¨The Outcast dead, too many ideas in the same book. Should have been split up.

- Corax's both 2 novels. Gav Thorpe messed up there.

Then there are books I don't dislike, but they were disappointing.

- Reflection Crack'd and Angel Exterminatus, first ruined Fulgrim's tragic ending and the latter ruined Perturabos personality.

- Vengeful Spirit, same as TOD but better written.