If it is low detail enough to consistently 'work', it isn't complex enough to be better than something like a chip and pin approach.

They are repeatedly bypassed with easy hacks like silly putty and photographs. People's biometrics are not unchanging. Burned fingers, swollen eyes, and sore throats are things that can change enough to make biosecurity unreliable. That is before cold and heat and how they effect biological things!

That is all before you take into account the fact that some people don't have whatever is being used. Have fun using eye based biosecurity on someone with cataracts or is missing their eyes entirely due to injury or just being born without them fully developed. Or they have a physical issue that makes it hard for them to interact with the bio reader. Stephen Hawking needing to lean towards a mounted eye scanner would be impossible for example.

So either you have mediocre security that allows for a lot of false positives to get through or you end up having to add a bypass system for when it fails, and now you have two ways that security can be defeated! A non-biological solution with two factor authentication of an item and a PIN or other knowledge piece is far more secure than biosecurity can ever be.

So already insecure, but in addition to that anyone with physical access to the person can force them to do the biosecurity. Police are able to force someone to put their finger on their phone, or look at the screen for a face unlock. Maybe they aren't legally able to, but it is a good example of not being secure.

They aren't 100% reliable and it has its' challenges based on its implementation but I wouldn't consider it fundamentally insecure. It's as secure as a NFC token, TOTP, or a push notification as a form of authentication. It's like birth control, no method is 100% safe and effective, but plain username and password auth is like pulling out, anything is better than that.