Apart from the obvious issues with biometric authentication (like the fact that revoking them is quite onerous and the fact that they are actually detachable) I have never really been much of a fan of them for other reasons, like the issue that they always seem to be fooled by low-tech means. A recent scientific study seems to validate that. Of course, I think the Chaos Computer Club video is much more fun (it is in German, but you'll get the gist of it even if you do not understand German).