More generally, you should have two keys for any lock, so you can still get in if you lose one of them.
* Lost access to your Google account, but still have access to your Apple account (or vice-versa)
* Apple device doesn't support Google passkeys, or vice-versa
But it multiplies the following downsides:
* Apple/Google account gets hacked, hacker gets all your 2FA credentials
* Snooping on your activity. Particularly Google, but Apple also have an advertising business.
* Setting up accounts on new sites is twice the hassle.
* Too complicated for the kind of folks who need the phishing protection passkeys provide.
More generally, you should have two keys for any lock, so you can still get in if you lose one of them.