No "cloud" storage service that actively syncs with a folder on you local machine is completely safe. Calibre does not support multiple users accessing its library files. A running calibre counts as one user and the cloud services sync daemon counts as a second meaning evil things can happen.
The three big rules for surviving with a library "in the cloud" are:
- Never rely on just the cloud copy and the local copy in your sync'd folder. Keep a complete backup copy of your library in an unsync'd location. Evil things can easily happen and leave the library unrepairable, at least without massive effort. A clean backup is very very very desirable.
- Never allow syncing while using calibre.
- Never allow multiple copies of calibre to access the same library at the same time.