BOF your spot on. The cu's effectiveness is directly related to the ability of the pitcher to challenge and locate with his heater. When hitters are forced to sit hard on a pitchers fb because he has one that they have to sit on to hit the cu becomes the great equalizer and is "killer". When a pitcher does not have a fb that forces hitters to sit hard on it in order to bang it the cu is not nearly as effective.
The best pitch in baseball always has been a well located heater everything works off of that. Folks its the reason velo is so important. It forces the hitter to speed the process up.