For reasons which I won’t re-iterate yet again, I use Sync.com for projects I share with my collaborator, and now for pretty much all my Scrivener projects. Despite the fact that the people at Sync.com say it is not really suitable for multi-file packages like a .scriv project, it has worked flawlessly for me over about 7 years … my only suspicion is that it might be a tad slower than DropBox (which I also have but no longer use for .scrivs). Furthermore, Sync.com doesn’t have the device limits of the free Dropbox.
My backups are zipped and they go to iCloud. On top of that, I make a full back up of the iMac to an external every week; I run Time Machine on the MBA weekly when its external is plugged in, though I’m thinking of getting another drive that I can do a full backup to every week.
Mark