I have suggestion about synchronization behavior that can solve some issues with adding new instalations of Operas to your account. If you add new instalation of Opera to your account most of a time you also add a lots of stuff you dont want like bookmarks that from new Operas instalation. Most of a time you probably dont want to add bookmarks from new instaltions but sync only your old bookmarks to new instalation and delete the rest.
So there should be option that appear when first time account login. Something like: Merge sync data, Override sync data from cloud, Override sync data from this instalation.