We had originally planned to go all-in on passkeys for ONCE/Campfire, and we built the early authentication system entirely around that. It was not a simple setup! Handling passkeys properly is surprisingly complicated on the backend, but we got it done. Unfortunately, the user experience kinda sucked, so we ended up ripping it all out...
The only way I ever used passkeys is with bitwarden, and there you are sharing them between all bitwarden clients.
From my very limited experience, pass key allows to login faster and more reliable compared to letting bitwarden enter passwords and 2fa keys into the forms, but I still have the password and 2fa key stored in bitwarden as a backup in case passkey breaks.
To me, hardware tokens or passkeys are not there to replace passwords, but to offer a faster and more convenient login alternative. I do not want to rely on specific hardware (hardware token, mobile phone, etc.), because those can get stolen or lost.