Richard Bourke you can run flow against classic libraries but the classic UI doesnt have the command bar integration. You could manually trigger the HTTP POST by looking it up in the Flow designer and adding in manually to the Ribbon.
Deleted you can copy files back and forth from Box to SharePoint today, but I;d hesitate to call it backup. If you have automated processes on one side, and you edit/empty a file there it gets the same result on the copy. Backup usually implies recovery, which you dont get with just file movement. The princiaol thing you are guarding against in the unavailablity of Box vs. SharePoint Online - and both, as entrprise cloud services have pretty high availablity. Today, emhaisze, today, if you have a flow triggered on all new documents in a library the event 'listener' is scoped to a single folder scope, or the root without subfolders. This is expected to change soon so you can have a Flow autotrigger for any file in a library. On-demand flows can be triggered anywhere, regardless of folder.
Dean_Gross a Flow can be used against any library - but the UX interation is in modern and the Content Organizer is a classic experinece.
Thanks for all the feedback.