I've found them to be a real pain in the arse because they're implemented so inconsistently. Only the biggest sites are offering them, but it's those big sites where I'm worried about locking myself out because of setting it up wrong.
I've locked myself out of Squarespace by setting up then subsequently removing a passkey. Doing so triggered a bug which "updated" the TOTP (that was already set up) and the backup codes. Support was absolutely deaf to the whole thing being a bug, absolutely impossible to report, and I'm sure it'll keep being an issue for years to come.
