The issue with Biometrics, atleast for those in the US, is that they are not protected by the 5th amendment. Court cases have reaffirmed this, meaning you can be forced to provide your Biometrics for law enforcement to gain access to your devices.

Some may feel this is acceptable with the thought if your not guilty then you should have nothing to hide, but this is a fallacy as it tramples over our 4th amendment rights. With passwords locked in your head, you cannot be forced to give that information as that is protected by the 5th amendment. Accessing the device or any information protected by a password must then be done with other means or not admissible as longer delays violates due process.