Earlier quoted context omitted.
That's a pretty long scroll box to find the person in the first place. Or an added step as you key in the first few letters of the person's name. At large retailers or QSR chains that put a premium on fast checkout, neither option will fly. I obviously appreciate the simplicity of paying with your face.
I doubt any firstname+lastname (or even just firstname+last_initial) situation would require a scroll box... you could fit 20+ recognizable faces on a screen. Sort them by age and in the few cases where there are that many collisions, cashiers would know just where to look. Keying the name is a bigger issue... so I suspect terminals in larger retailers will eventually just detect the N nearest pay-eligible handhelds.
In contrast, barcode scanners are already there and they typically emulate keyboards, so no special integation is required.
I'm not convinced that paying with your name scales for most retailers. The retailers I've talked to aren't convinced, either.