>>caust1+(OP)
There are some hairy edge cases during registration that many get wrong. (At least GitHub and google had this bug) that if create() returns but the passkey never reaches the server due to bad networking conditions that your password manager thinks it can log in but the server never recorded the passkey for the user. Basically there is no transactionality and you can get in a split brain situation where your password manager and your server don't agree and it's very confusing for end users.
https://github.com/w3c/webauthn/issues/2038
They apparently came up with a fix for this using something called Signals API but I don't think any browser implemented that yet.
Just wanted to highlight that this part of the UX is hairy and hard to get right