Biometric security is implicitly insecure. You can easily duplicate someone's fingerprint, face, voice (just added to Lollipop), or iris with a photo or recording. The only way to make it secure is to have another person next to the machine verifying that it's a living, breathing person whose biometrics the device is reading, not a photo or recording or some other duplicate.
Also, I'm skeptical about the reliability of iris scans. I was in the government's NEXUS program (fast lane between the U.S. and Canada) back when they used iris scanners at the airport. It works well for some people, but is problematic for others. I always had to take off my glasses, use my fingers to hold both my eyes wide open, and the machine would still take 3-5 tries before it recognized me. My experience must not have been unusual because they've since gotten rid of the iris scanners and replaced them with fingerprint scanners. I've never had a problem getting recognized by those.