I think that's the key distinction here. In any given authentication scheme it's important not to have false positives (incorrectly identifying a bad guy as you) or false negatives (incorrectly identifying you as a bad guy). In this case false positives break security, false negatives break usability. However, false positives won't outright stop adoption whereas false negatives will.
With glove, dirty or too much sweat, I believe it does not work. So, it's not 'always'.
However like others, I turned it off because the performance was highly variable, and the failure mode consists of a many-seconds wait which can be extremely infuriating (even embarrassing, as as you stare blankly at your phone for 5 seconds at a party, trying to quickly get someone's number or something).
edit; not 911emergency, but casual situations of full or dirty hands..
I went to it. I leads to a special dialer. Instead of voicemail the button leads to a special emergency contact (or list). It only shows 4 inputs on top so I am guessing that is the limit so you can't dial anything but emergency services (that are 4 numbers or shorter). Then it goes back to my lock screen.