How dangerous is --inplace

[A B]

Hello list!

How dangerous is acctually the --inplace option if you want to run
rsync to update files that are only  read and not written to? What is
the worst that can happen? The file is not readable, the reader gets
half the file of an old version and the rest is from the new version?

There is not a simple way to make it update the file in the standard
way and then change the ownership afterwards, or is there? Do you have
to write that program your self?