As far as I can tell, biometric authentication is locked to proprietary operating systems. On Linux with a yubikey, for example, it seems like you're not only limited to only 25 sites, but you're also at best going to have a pin, and in many cases the hardware alone may be sufficient to gain access. Sure, you need to know what site the key has been registered with, but I'd bet if you found a random key at a conference you'd have pretty good luck trying it with google and github to start with.

edit: after some digging (which was a lot more involved than it should have been) it seems like the current state is:

There is free software to set and manage a pin for a yubikey on Linux. Firefox historically didn't support yubikeys with a pin, but it seems like that was recently merged. Yubikeys still have a 25 site limit per device, and no sync across devices. As long as sites let you register multiple yubikeys as a backup, and support pins, then it's a reasonable workflow. I'm not convinced it's better than passwords + a yubikey for 2fa, but it seems like in practice it's probably not worse either. It still feels like, even if security is a motivator here, there's a lot of opportunity for Google, Apple, and MS to conveniently and "accidentally" cut free software users out of being able to access a lot of the internet with the move to passkeys, and I remain skeptical.