C.S. Goto is definitely the worst author in the entire Black Library, although Henry Zhou is a strong contender.
Dan Abnett and Graham McNeill are the two heavyweight authors in the Black Library, Aaron Dembski-Bowden is a rising star. Honestly, the only authors you really need to avoid are Goto, Zhou, and Gav Thorpe. Ben Counter has written some real shitfests, including what is arguably the single worst book of the entire Heresy series, but the Soul Drinkers series is fantastic imo. Basically, aside from the ones I mentioned by name, Black Library authors will generally deliver exactly what you'd expect for a sci-fi novel written in a pre-existing setting.