Don't know about face id, but with finger ID you certainly need to know the pin number for all the cases that finger ID doesn't work.

This is the case with Face ID as well. I use my PIN more with Face ID than I did with Touch ID.

More than that, it's easier for a security guard to point the phone at your face / push your thumb on the sensor than get you to reveal a passcode

It's fairly easy to discreetly disable, FWIW. Just "squeeze" your phone for two seconds (i.e. press the power button and either volume button) to disable Face ID. It also won't work if you're looking away or have your eyes closed.