When you use
apt-get without a no-action modifier, it means you are going to perform some actual operation with packages like install, remove, upgrade etc.
-q means quiet. It produces output suitable for logging, omitting progress indicators. But when you use
-qq which is quiet level 2, it implies quit with
--assume-yes). In this case no output will be produces except error.
Now why it is considered as dangerous?
apt-get removes some packages to satisfy the dependency. Using
-qq in such condition may remove some package which is important for you or for the system without informing you.
So it is not considered as safe because such operation can make your system unstable.