Matter
Absurdly lengthy, with one of the slowest setups ever followed by an equally abrupt and unsatisfactory resolution which kills off pretty much every character we might have even a faint interest in† thanks to something which is introduced out of nowhere maybe 4⁄5s of the way through. Banks is a great author, so of course there are plenty of rewarding nuggets scattered through out (the haunted ex-Culture man, the deceptive artifact, the general convincingness of ‘the stage is small but the audience great’ theme etc.), but much less than expected from a groaning tome’s worth of Banks. The shell world concept is not developed or employed well (you could replace all of the maneuverings with 2D equivalents if you wished), and the rest of the background and concepts seem pretty stock Culture.
† eg. I was expecting the betrayal to be linked to alien machinations, but no, it turns out to be exactly as simplistic as it seems, the ex-SC guy’s good argument to the contrary, and a complete red herring as he dies with everyone else, the complexity of his character never ascending beyond cackling evil.