The sync feature seems to be purely based on trust and there is no way that i can see to know what data has been backed up for sure.
Chrome gives you the option to see what has been synced.
I personally would be happy if the sync screen just showed some totals such as number of bookmarks synced etc. Just so you know for sure that your data was backed up
It would be nice to have this functionality…but brave works fine by the way.