I audit these for a living, so: what goes wrong is that a meaningful fraction of the findings are true statements about the DOM and wrong statements about the product.
The ones that show up in nearly every suite I see:
- Contrast reported on controls that are disabled, where the low contrast is the meaning.
- Decorative imagery flagged for a missing name, where a name would make it worse for a screen reader user, not better.
- Duplicate landmarks reported inside a modal, because the checker sees the page underneath as well as the dialog.
- Findings on third-party widgets you do not control and cannot fix this quarter.
None of that means the tool is bad. It means the tool reports what it can see, and whether a finding is a defect depends on intent, which is not in the DOM.
So the failure mode of gating on everything is not false confidence, it is abandonment: the gate fires on something nobody agrees is a bug, somebody adds an exception, then a wildcard, then a skip, and within two sprints you have neither the gate nor the report.
What survives contact with a team is smaller: pick a short list of rules your team has actually agreed are never acceptable, fail on those, and attach the rest so the trend is visible. Grow the list when a category stops appearing.