Important to note that in the Culture universe all the extant AIs are _explicitly_ noted to be psychologically flawed in some way, with every culture that's managed to produce a true ‘perfect’ AI immediately ascending to some godlike plane of pure energy, whence they interfere in the affairs of meatbags only in the most dire of circumstances. So it's surprising to me that Banks has said he considered it a utopia given how explicitly (in my reading) he made it a non-utopia in the books.