On smaller flights, there may only be one gate agent and scanner. If someone's boarding pass isn't correct or there's some other issue, there gets to be a big backlog while they get sorted out. Sometimes that takes several minutes, and the smaller flights are often the ones with short 20-30 minute turnarounds scheduled.

Had this happen on a flight I took a few weeks back.

And that's still going to be the case with facial recognition.

The only time saved is scanning a ticket vs scanning a face. And people are pretty much guaranteed not to lose their faces.

Not if it's an automated gate.

I just got back from Australia, and the passport gates there are automated facial recognition; they look kinda like the gates in a subway where you scan a travel card.

A reject heads over to the desk to get sorted. Everyone else keeps going through the automated gate.