Settings, but you'll miss out on other, well, settings, too if you disable them.

Actually, none of your devices will be a source - everything gets merged together so you will both have your desktop bookmarks on the mobile device and the other way around - Opera's Sync acts roughly the same as Chrome's one when it comes to the overall behavior, and honestly every other browser synchronization system that I know of.
I can assure you that nothing will get lost, that's my job to make sure of it ; ]
However, that's a valid point if you feel that the documentation is not deep enough for 'more technical' users, I'll check what can we do about it.

Wait, you're suggesting that there should be an option such as 'Remind me what my passphrase is'?
If so, then it's absolutely impossible from the technological point of view - it's not anyone bad will, it's cryptography (pure maths actually). These passphrases aren't actually stored anywhere - they are used to encrypt your data before sending them to Sync servers, and Sync itself has no idea what that passphrase could be.
Think of a safe unlocked by a numbers combination - there's no way to know what that combination is just from standing before the safe, unless you completely break it and see what is inside - and this, in case of modern digital encryption, would take a few dozen centuries at least.