Earlier quoted context omitted.
Among other problems, Safari keychain backups don’t work unless you store them in iCloud (and there’s no warning about this): https://news.ycombinator.com/item?id=24626562
This does appear to be sorta documented: Note: You can’t copy passwords stored in your Local Items or iCloud Keychain. To transfer these keychain items to another computer, set up iCloud Keychain on the other computer using your iCloud user name (normally your Apple ID) and password. You can manually copy keychains other than Local Items or iCloud Keychains to another Mac using the steps below. https://support.apple.…
Either way, I'm not sure this counts as a warning! It rather reminds me of:
As you are probably aware, plans for the development of the outlying regions of the galaxy invoke the building of a hyperspace express route through your star system. And your planet is one of those scheduled for demolition.
(Shouts of terror emit around the globe.)
There's no point acting all surprised about it; the plans and demolition orders have been on display at your local planning department in Alpha Centauri for fifty of your earth years. If you can't be bothered to take an interest in local affairs, that's your own lookout.
Most people setting up their keychain without iCloud will never see this, and there's not even an explicit mention here of the issue with backups (though one could sort of infer that).