But there are genuinely things that can't be feature detected. A bug that happens 1% of the time due to reasons still unknown, or features or bugs that need so

But there are genuinely things that can't be feature detected. A bug that happens 1% of the time due to reasons still unknown, or features or bugs that need something to happen on the user or hardware side to detect. For example, I had an issue once where when using WebRTC on some devices with 3 rear-facing cameras on android it would horizontally flip the "long range focus" camera. There is no way to feature detect that. Hell there wasn't even a way for me to feature detect that I should use the second of the 3 cameras on that device (the first was a fisheye-style wide angle lens, and the 3rd was a middle-ground lens, but the second worked best for our usecase). I've also used UA detection on a browser bug that would cause fetch requests to fail if the user went offline for a split second inbetween requests. There is no way for me to detect that before the bug happens, and once it happened the "failures" looked like it just never came back online. UA detection was the only tool I had to fix that until it was fixed at the browser level.