The cover has a blurb from Mike Mignola: "If Jules Verne and George Romero got together to rewrite American history, it might go something like this." Of course that sounds like something I'd love but as you've certainly guessed I didn't. Priest set out to make an explicitly steampunk novel (in fact "steampunk" is the final word in the entire book) but it reads not just like a first novel but like it was written by somebody who had really only even read a few novels. In fact Priest has published a handful before so either she's just that bad or simply had a dip. A good editor might have salvaged a little more but those are an increasingly rare breed (though the more scary alternative is that the original manuscript was worse and the editor did indeed work on it).
Just look at this way of providing background. The book opens with several pages from a (fictional) non-fiction book that explains the history of this alternative-history. Then the first part of the actual narrative has a biographer show up to interview the main character Briar about the person he's writing about so now we get more backstory. A bunch of exposition, explanation of characters and then the biographer vanishes until a pointless epilogue. That's a lot to slog through to get the actual story started and it could have all been easily cut out with bits and pieces reinserted elsewhere if needed.
Once the story is going, Priest proves to be a lackluster stylist. She favors a clean, almost blunt approach as so many current writers do but she also can't help but push readers. "Without the coat, her body had a lean look to it--as if she worked too long, and ate too little or too poorly." Priest has already told us Briar works too long and is about to get to the eating so this is just heavy underlining. Characters talk while raising eyebrows, under the breath, mumbling, agreeing, etc - not to the point that it's obtrusive but more than necessary. Considering her steampunk intentions a more Dickensian approach wouldn't have been out of place or failing that some good old-fashioned pulp excess. Admittedly both are fairly out of fashion except for parodies or deliberate pastiches but this would have been the place.
The story itself isn't completely hopeless but is a bit more straightforward and predictable than needed. Priest even makes the three most powerful characters women and then has one of them point it out - fair enough but readers already noticed and if they hadn't then this only jostles the story. Too much is a character having to get from A to B and then the main suspense is "will they get from A to B?" Sure, that is more or less true of The Odyssey or The Divine Comedy or Lord of the Rings but the difference is whether A-to-B is all that's going on. And too much for Boneshaker it is.