Don't have any paper from Apple about this, but first finger-print scanners started to check temperature to avoid reading old fingerprints left on the reader when someone did breath on it to trigger a new reading. So to attack the temperature check you had to place a plastic-bag with body temperature water on the reader, making it read the old fingerprint if enough of it was left on the reader. Then they started to check for pulse, so the easiest attack is to put a false fingerprint on a finger.

Fingerprints are easy to copy and you cannot change them, so fingerprints might be more secure than use the pin 1234. But it isn't more secure than a strong password, but so much more convenient to use...