Pretty underwhelming if that's the future of web auth..
https://www.zdnet.com/article/best-security-key/
https://www.wired.co.uk/article/best-security-keys
Or here's an actual expert writing about them a few years ago if you suspect "tech press people" are worthless:
https://www.imperialviolet.org/2018/03/27/webauthn.html
Or, more practically, if you have a nice phone, try visiting it on that. If it's a relatively modern iPhone or high end Android with a touch sensor running the current OS version you can use that.
Since you mentioned Mac keychain, if your Mac has TouchID that will work on the current OS (in Safari at least) too. Some Windows PCs with fingerprint sensors likewise.
WebAuthn doesn't end up with a third party (say, Facebook) having biometric data, what they've got is just a public key and an identifier. Your device is signing to say it checked you are still you, whoever that is. It does not promise how it did that and there's no reason a web site would care.
Your biometric data (if that's how you authenticate) is only needed by your device, to verify this is still you when it makes that claim. So any changes (e.g. you decide to use your other hand) are a local device problem, no need to tell any third parties anything interesting happened.