This post goes over nested folder sharing with Sync. It makes sense but it limits the redundancy you can have with a network of Sync devices.
Introducing a brief overview of how I use sync (image attached). I use a RaspberryPi to house absolutely all my files from photos to music to documents, I keep everything in one folder called 'Documents'. This folder is mirrored between my laptop/desktop/and RaspberryPi and it works great.
The problem is when my RaspberryPi goes down. My laptop and desktop still mirror each other, but they don't get mobile phone photos and phone #2 doesn't get mu