One problem with using biometrics as an authentication mechanism is that mere presence is not authentication. Aside from some more gruesome science fiction stories – does the finger with the finger print need to be attached to the rest of the body – there is also the case that just because the finger touched the sensor, it does not mean that the person intended to unlock anything.
Another problem is environmental, when it is -40 or below, who wants to touch anything? Another case is sterile environments – you do not want to touch anything with bare skin after scrubbing up. A related problem exists in industrial environments where hands might be exposed to paint, ink, oil or any of a wide variety of other substances that make reading a finger print unreliable.
Denial of service is also a problem in cases where the relevant print is damaged or hidden due to injury.
Overall, biometrics might be a possible solution for some extreme situations, but for the run of the mill unlocking access to most real life transactions, they do not provide the necessary intentional action or ease of use.