I encountered the same issue. I use Opera GX on my macbook and now, I want to use it on my iMac. I installed the GX and login, do the stuff for synchronize but it doesn't sync all of the setting (sidebars, flows, mods etc.) and bookmarks to each other. What I got is that the GX synced the tabs I opened in macbook to iMac but other than that...non is synced.
Hope this problem will be solved soon.