They've been around for quite a while and a quick Google of the problems with such devices will make you aware of some of those issues. Knowing that fingerprint and iris scanners can be hacked raises the biggest problem, in my opinion. Once hacked, passwords can be changed but how do you change you fingerprint or your eyeball when it gets hacked.
For ideal security, most security conscious companies use three different methods to secure an account, etc. Password first, then biometric, and then maybe a access card scan just to get in the door.
Hope this helps.
There is a device that can be purchase to attach to a computer to place your finger on to identify you as the owner of the computer. This would be a better than a password, 4 digit code, or a picture code. It only cost $39.99.