Honestly, I'm very surprised extension sync isn't yet a feature. The major browsers support this feature (Firefox and Chrome); the way I see it, that's a pretty big drawback of Opera for those that have more than one device!
It's something easy as extension name and config files? 5Mb per user? 20Mb max maybe?
Actually full Opera back up cloud sync? Dropbox - Drive - OneDrive - iCloud? - Mega - SpiderOak...
How many users in this forums have problems just with the bookmarks?
They even do not know about Opera Sync, do they use single folders bckp soft?...
If Opera do not plan to implement full browser sync, for whatever reason, it could be very good to advise users to use their own cloud sync software, may not the best but better than read lots of post with people complaining about, in the worst case, data loss.
Lets see with the Chinese people if there's good news.