I understand how users register and login on a device - but it gets complicated when wanting to allow users to couple multiple devices to an account.

Anyone here with recommendations?

Say you registered via smartphone and now want to login on the desktop - do you tell the desktop user to grab his mobile safari, login and pull up some PIN-No which then to type into the desktop client?

And how would people recover accounts if their devices are lost?

I guess technically one can always come up with some solution - but while FIDO gives a unified, cross-device way for users to login and register – it’s the complete opposite when it comes to the aforementioned issues