What Harry said.
(Background) Delays are bad.
What if Calibre does 2 consecutive near operations on a book file? And one of those takes longer to propagate to the cloud? What if something on your network also dumps a huge file to the 'cloud' at the same time?
Building in all the necessary features to make everything 'Transactionally safe' would severely impact speed (And I see howls over current HD limited speeds here all the time. It took 10 seconds

to transfer a 100 books to my X !)