this doesn't sound like the passkey is the issue, is the implementation or the missing implementation issue.

in poland we have similar to passkey implementation for government profile, that is then used to login to most/all government websites or to sign government documents. you point the camera on the qrcode, confirm it on the phone and you are done. this same app has your ID, which can be used in most places (shops, banks, police etc).

and btw im using linux (main box), macos, android and ios - no issues so far with really cross device usage