FWIW, I am of the opinion that it is a software issue, at least the one that impacts me specifically.
I get ~90% success rate. When it fails, it does so NOT with a "try again" but as if it doesn't sense the registered finger and eventually moves to "slide to unlock". HOWEVER, if I put a non-registered finger, it correctly provides the "try again" message. So clearly the hardware is sensing something.
It rejects incorrect prints from a non-registered finger, but struggles to fully accept a registered finger 10% of the time. It's as if there is a third option – accept, reject AND "not really sure".
When in the "not really sure" state, it never eventually accepts, but 100% of the time I can get it to work normally when invoking control center and opening and closing calculator.
Incidentally, and I didn't have the phone long enough to say for sure, I started to have issus after the update that added the slight pause before the "slide to unlock" message appeared. Originally, it appeared immediately. Certainly not diagnosing anything by saying so... just curious if anyone noticed the same thing.