What are you hoping to sync? If it is just your subscription list and every device fetches by itself it shouldn't be too hard. However more complex things like read/unread would be hard to sync using a basic tool like Syncthing. The problem is that tools like Syncthing don't really have any conflict resolution capabilities so apps would need to be written specifically for that form of syncing.
Most open source readers use a shared backend server that manages checking feeds and syncing read events rather than this filesystem level syncing.