The books do have a strength, which is the completeness of their picture of the world. The concept of the Culture is interesting. I think it has less to do with the third dimension of space, and more to do with the 1990s. The Culture looks a lot like confident liberal democracy, pitting its advanced tech and cultural openness against its various (doomed) rivals. That's why a lot of the books seem a bit colonial: travel to a small annoying planet, knock some sense into the primitive authorities holding sway. And it also has that 1990s vibe that one side is in the right, and that side is always going to win.
For a contrast, think of The Three Body Problem. It's set on an equally epic scale - but so much more happens!