I have seen that codex -latest highest effort - will find some important edge cases that opus 4.6 overlooked when I ask both of them to review my PRs.
I did notice multiple times context rot even in pretty short convos, it trying to overachie and do everything before even asking for my input and forgetting basic instructions (For example I have to "always default to military slang" in my prompt, and it's been forgetting it often, even though it worked fine before)