I've read this book twice. Unlike a couple of other reviewers, I found enough to interest me that I bought the rest of the series.
Scar Night has a lot of possibility. Campbell has built an intriguing, steampunk-ish, 'is it F or SF?' world around the city of Deepgate, suspended above a truly deep and dark abyss. He's put interesting people in the world, and the story itself was strong enough to leave me wanting answers to some of the mysteries.
Unfortunately, Campbell also skimps on some of the scene setting. Even after the second read-through, I'm still not entirely clear on how the city is suspended. Campbell spends quite a lot of time talking about chains, ropes, and rings, but it's only very late in the book that we get much in the way of helpful description. So I spent much of the first reading trying to figure out what was where, what all the chains connected to, and why.
The story is similarly opaque on a number of other fronts. The uncertainty only works in the story's favor on the F or SF angle; the rest of the time it's more frustrating than intriguing. (An exception is the deliberate mystery about the base of the abyss. The reveal is a bit of a letdown, but a few pages into the sequel, Iron Angel, that may improve.)
Deepgate exudes a very Gormenghastian feel, but it's not clear to what end. The writing is not as polished as one might hope. Still, I'm happy to say that it does improve on a second reading. There's quite a lot of detail - if not quite world-building, at least world-ornamenting.
I can recommend this for fans of Mervyn Peake and steampunk. For readers who have trouble working out the engineering, as I did, I recommend looking at the above spoiler, and letting it go at that.