For example, the Edge bug was at [1]. It didn't reproduce when the devtools were open, it only happened if the optimizer tried to optimize it which was non-deterministic, and it would only trigger in some cases in pretty weird spots with a lot of supporting code.
Feature detecting that would be a nightmare, but UA detecting it (since I now know the exact versions impacted) is super easy and simple and the fix is extremely small. I don't have that in our codebase any more, but it was a perfect example of a bug which can't easily be feature-detected.
The problem is you can't reliability detect non-deterministic bugs, so in those cases you essentially have to use the UA to decide if you can use them.
