Why most auto-merge gets switched off
Rule-based auto-merge looks at green checks and nothing else. A green check doesn’t know whether the change touched billing or fixed a typo, so teams either live with the risk or switch the feature off. CodeReviewer looks at more before it merges: a full review of the change against the whole codebase, your test results and, if you want it, a match against the spec your team agreed to build.
Humans stay in charge
Auto-merge is a convenience, not an override. If a reviewer disputes a finding, if a condition isn’t met, or if the policy is off for a repository, the pull request simply waits. CodeReviewer’s review and approval sit on the pull request like any reviewer’s, so anyone can see what it checked before the merge.