> Not "works" in the sense of keeping bad people out, but "works" in the sense that when I use it my phone unlocks.
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.