About a month ago, there was a report on potential issues with iris scans. This week's Frontline contends that fingerprints are not infallible either and the related ramifications for the UID/Aadhar project. It also mentions that Nilekani has publicly stated that iris scans are "not a mature technology" and has questioned its accuracy.
An article in Nature points at research claiming that human irises are not constant over time and instead change appreciably as we age. In a paper by Kevin Bowyer and Samuel Fenker, they assert that there is a noticeable change between iris scans taken only three years apart.