Why is usb 3.0 64GB flash drive not recommended to back up my photos on MacBook Air?

Quoting the Apple Support document you referenced... "to prevent data loss."

Nothing is preventing you from using it. Just be prepared for data loss, which can be said for anything of value. To preclude that possibility, use a backup method such as Time Machine.

https://support.apple.com/guide/mac-help/alternatives-for-backing-up-your-mac-mh 9052/10.13/mac/10.13

My understanding is that flash drives use a cheaper chips over the chips found in an SSD.

Library of Congress has not done long livity test on chip devices. They recommend the tried and try hd

's.

R