I looked at those apps briefly. Below are quick impressions that @taxedserf might comment on.
I’m just a Duplicati user, and quite new to a lot of this, but this is an interesting topic to consider.
What issues (besides disk use) does one run into simply backing up Backup and Sync’s drive?
Minus conversion, BackupGoo sounds like it just gets its updates some way other than on disk.
Duplicati now uses some OAuth, but I believe it’s only to get to destinations to do simple things.
Spanning runs at Google as an app. It can probably get to Google Drive fast, and AWS less so.
Interestingly, I see that Duplicati already uses App Engine, and App Engine allows C# and .NET.
Having said that, a lot of ideas have merit, but I suspect the focus is currently to get out of Beta.
Thanks for the suggestion, and perhaps more discussion will follow.
Meanwhile I wonder if rclone with B2 versions or --backup-dir can help as some sort of backup?