There are several minor criticisms I could level against Company of Slayers, the prose is a little weak (albeit not terribly so) and the plot is inconsistent with its tone (I.e the plot reads like an adventure story, but the plot itself is simultaneously largely humorless and a straightforward action narrative.) Ultimately though, the core flaw of the book lies in its characters, who are plagued by multiple issues. For the most past, they run the gamut of boring to unlikeable, with the non magical main characters being little more than melee fighters, while the magically gifted characters are so vastly more powerful and competent it renders the ungifted characters almost entirely irrelevant. They are a group and yet there is no balance of power moments, no spread of skills, no tangible element to promote unity within the group besides happenstance. Compounding this is that the group barely interacts; there’s no camaraderie, no companionship, barely any sense that they even like each other or grow close through their trials (several of the characters actively considering abandoning the rest of the group in perilous situation, even unto the final climax of the book.) This lack of interaction also prevents the characters from displaying themselves, so that the only way we get to the know the characters is what they do in action scenes (which for the ungifted characters isn’t much more than hit things.) Even your more complex characters like Slinker, Dryana, and Saliana are barely allowed to express their complexity, or their personal narratives explored. They’re just touched on and then moved past without ever noticeably affecting the plot. ( With some minor exceptions.) The result is a book that’s largely superficial, with little to invest in emotionally, driven primarily by the action scenes (some of which are good.)
The world building is decent though, with rare magical crowns that allow a magically gifted individual to meld with the ships (and which actively influence and drive the plot), ships that travel beneath the ocean surface, a variety of different species, threats, and enemies.
There’s a lot in this book, it’s just not cohesive; the book feels like an adventure some, sometimes even like a DnD inspired adventure novel with wererats, ship hijacking, dungeon-esque escapades, and the party of adventurers plot, but it just doesn’t feel like it was meant to be ‘fun.’ It’s grim and serious (one of the main characters is gang-raped in the first chapter) but without the character depth, complexity, and narrative you need to make that work.
As above stated, the book’s flaws are almost all rooted in the fact that it’s characters aren’t likable, either morally gray without power moments, or morally gray without humanizing moments.