People who get headaches have wondered if it was Face ID, but I have not seen 1 case where someone was able to trace their headache back to Face ID 100%.Somewhat lower security, as your iPhone can then be unlocked without you looking at it, even with your eyes closed.
With awareness feature on, you need to actually look at it in order to unlock. Apparently, this is when our eyes get bombarded with extra rays, potentially causing headaches for some people.