I use SyncThing [0] to sync my anki databases between a laptop, desktop and a raspberry pi. Fingers crossed nothing has gone wrong yet, apart from problems such as using incompatible versions on the same db.
Thank you for the suggestion - as a med student, she has an iPad that can access certain medical resources, and is already an iPhone user, so I don't know if SyncThing is really an option for her.
I'm not sure that most users can be expected to figure out things like "does this update include a database upgrade?" "which devices are running an old database?" and "what do I do when the software told me the database on disk is corrupt?"
And I think SyncThing would still suffer from the issue of needing to make sure only one device writes to the Anki sync at a time. I really think some sort of cloud service that handles multi-headed sync of cards/notes (and makes it hard to take permanently destructive actions) would be a real competitor in this space - if they can overcome the Anki network effect.
[0] https://syncthing.net