I have been using CC with Dropbox for a very long time. The previous developer was very responsive and fixed bugs very often. The new developer didn’t have the competency to keep up the user needs. Now the application is basically dead.
I switched to
Calibre Sync and I was quite pleased with both its functionality and its responsiveness. I highly recommend it! The other thing I like a lot about this app is that it can sync multiple libraries even on different clouds.
For people that look for a free app with similar functionality, for a while I used
Libreca. It was working decently, but was only for iOS. Unfortunately, it is no more available on the App Store. But if someone want to pick up where the old developer left it, the code has been made available on GitLab:
https://gitlab.com/calibre-utils/Libreca