If I recall correctly several medical conditions can be detected by retinal scans, which has historically made widespread use of the technology undesirable.

EDIT: Sorry, it seems that it's more that medical conditions make it unreliable

> Certain eye-related medical conditions and diseases, such as cataracts and glaucoma, can render a person unable to use retina-scan technology, as the blood vessels can be obscured.

https://www.biometricupdate.com/201307/explainer-retinal-sca...