I'm skeptical about PassKey. I find KeyChain sometimes fails—websites won't recognize the password, or KeyChain and the website have lost their ability to interact. Thus even though my passwords are saved in KeyChain, I also always save them manually in a document on the encrypted drive on my Mac. When KeyChain fails, I look up the password in that doc, and enter it manually.
The problem would seem to be that, when PassKey fails, since it doesn't use passwords, you won't have a password to look up and enter manually.