It seems to me that it should be safe to use Dropbox, if you stick to the following regimen faithfully.
- do not use Dropbox for anything other than the Calibre library,
- turn off (or pause) automatic syncing of Dropbox on all computers,
- before opening Calibre turn on Dropbox syncing only on the computer using Calibre and turn off Dropbox syncing before opening Calibre, This will sync down any changes made on another device.
- do not open Calibre on multiple computers simultaneously, so only open Calibre while using it and close it (thereby syncing to the database) when finished,
- when Calibre is closed, turn on Dropbox syncing and turn it off again when complete. This will prepare the updates to all other devices.
Obviously, this will only work if only one person is ever using the Calibre library and it is a little tedious, but could probably be easily scripted.