> What I’m unsure about is whether diff-only context is actually sufficient for meaningful reviews, or if this becomes misleading without deeper repo and architectural awareness.

The results of a diff-only review won't be very good. The good AI reviewers have ways to index your codebase and use tool searches to add more relevant context to the review prompt. Like some of them have definitely flagged legit bugs in review that were not apparent from the diff alone. And that makes a lot of sense because the best human reviewers tend to have a lot of knowledge about the codebase, like "you should use X helper function in Y file that already solves this".