If I remember correctly (which may be wrong as it has been quite a few years), veins in an eye are better than iris for many issues related to using eyes for identification, but they can also reveal health information and so there are more issues with storing and sharing the data needed to verify a person.

Now, one may point out that if both pieces of information were being kept entirely private and were being secured with industry standard security this different wouldn't matter. I'll leave the implications of this for the reader to ponder upon.

I remember an Iris scanner demo from 20 years ago that worked because of the characteristic flutter / vibration of the iris as much as the pattern of the iris itself. As I remember it it didn't work well with high pigment eyes (it worked brilliantly with blue eyes, less well with everyone else).