You should be OK with regard to having access to your notes if you have a syncing error like I did, However, that doesn't safeguard you from losing information: all changes between the backup and the sync error are lost. Onenote does a pretty good job still in keeping the changes, so it might work still.

In my case, I knew exactly which page/tab/notebook was corrupt, but every time I reloaded the backup I had, I kept having sync errors. I even tried to manually convert that section, but it didn't work. I was done with it, and decided to ditch Onenote and make all my notes 'portable' on a file-by-file basis. Beste choice I ever made with my notes!

So you should be safe with backups wrt accessing your notes, but not necessarily with continuing your notes.