As for Simon Cowell, the way he expresses his opinions for whichever TV show can be very blunt and intended for the camera, but he is basically honest. If someone doesn't have the ability or qualities needed for the show (not necessarily the same as not being able to sing), he will say so. Some go away and try harder to prove him wrong. Others actually can't hold a not in a very large bucket, and he might actually be doing them a service to tell them.
Quack!
GtD.