As far as I can observe, there's not only me with that issue.
I reinstalled Windows 10 1803 and freshly installed Opera doesn't sync any bookmarks despite successful log into my account. Also there is no any Speed Dials synced but History is synced. So we have partly broken sync.
Web version at sync.opera.com/web shows there's all my data so it's not lost. But by some reason it's not delivered anymore on any NEW devices.
I have also Android phone enabled to account. It can see synced tabs on my desktop, it can see my bookmarks as it was always before but it CAN'T see Speed Dial of this freshly installed Opera. Only previous Speed Dials before Windows reinstallation.
I checked what if log in on another Android Device: as a result, bookmarks sync also doesn't work - shows empty. So, issue persists across devices.