But the Codex must update. Guilliman, whatever people think of him, was no fool. The treatises written by his traitor brothers on their specialist topics were never to be bettered, so he left them in, showing that he is, primarily, about practical knowledge. He distlilled all Astartes warfare into a seminal book, much like the Tactica Imperialis. And it's not just a 'do this and win'-type thing, but also there is a lot of Sun-Tzu type stuff.
What would Guilliman do if he had lived through the time between the HH and 40k? Without doubt he would have added to the Codex, reflecting the changes in enemies and the evolving role of the Astartes. So, what I think, is that the Codex now varies between Chapters. By this I mean that Chapters that worship the book, like the UMs, might never change it from the original version, hamstringing themselves with their reverance. Other Chapters are probably more pragmatic about it, adding in new essays and tactics as they are needed and appropriate.
I think this reverence for the Codex is a wrinkle of depth that people miss when it comes to the UMs. They are disliked because they follow this book and think they are better because of it. But isn't there an element of tradgedy about this? If they don't update the Codex, then, slowly but surely, it becomes more and more outdated as will they. They will be forced to try to gain tactical advice from discussions of past situations that have nothing in common with what is actually in front of them. How do they grow from this? Do they ignore it and become more and more irrelevant, inflexible and hidebound as time goes on, or do they, as a Chapter, abandon their beliefs in the infallibility of Guilliman's master-work? What would that do to them, removing a central pillar of their psyche? Would there be a schism, a civil war between those who think they have to change and those who think they just need to get better at reading between the lines? And what would happen to their Successor Chapters? Would there be a split in them, reflecting the issues within the UMs themselves?
There is a lot of depth with the Ultras, if we just think about them the same way we do about all of the other Chapters. Here is a group of heroes, just like all other Chapters, but one that is racing towards a crisis of faith in its own identity. They are only 1D if we think of them like that.