You may get an error when trying to select the root of an exFAT drive as a source or destination. This is an issue in macOS itself; it prevents SyncTime from creating a so-called bookmark that would allow it to keep access to the drive after relaunching the app. I submitted a bug report to…

SyncTime is agnostic of external cloud providers, and it only syncs files and folders visible in the Finder. Most cloud providers offer an app that allows you to mount your cloud files in the Finder, so that you can then sync them with SyncTime.