Devices that break or get lost, and then either the user had password authentication activated all the time anyway in addition to the passkey, or they have the classic "mother's maiden name" recovery path, or they get locked out of their account.

Brilliant security: a highly secure high-tech shiny front door that can randomly fail to open, so you still need the low-tech back door, which is the one potential thieves will use.

Mind you, it can work if the back door is old but sturdy (basically, for a bank that will ask for KYC authentication, or in the worst case you can set foot in the brick-and-mortar branch showing your face and ID and ask for access) but for pretty much every other service it introduces risks for very little or no gain.