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.
Haha! Wow, the part where there is a device down the pipe that acts as a mini laboratory, analyzes the fecal matter passing through, and sends you medical alerts on your smartphone… I could totally see this exist in the future. Identification could probably be done by sequencing the gut microbiome to avoid the camera taking pictures of your ass.
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...