This is a subject worth of a whole book by itself. At the ML level the CU can be the toughest pitch to hit. If kids just throw some every day with a CU grip they can learn to control it and find it can be a great pitch in just about any count.
As mentioned, hard throwers tend to shy away from throwing it because at the lower levels kids can't catch up and you are giving them a chance. Howe4ver that may be a win now vs development issue. At the LL distance the CU is the most effective second pitch since it's tough to get much movement on a breaking ball.
The CU is always thrown "OFF" the FB no mater what the count. It is effective because the speed is less then what is anticipated. The pure speed of a CU is not what is most important but rather the relative difference from the FB. I heard coach Murphy from ASU say his player's bats were tuned to a certain range. Pitches above or below that range gave them trouble. The challenge at every level is to maintain FB mechanics and arm speed but throw with a CU grip.
Lastly, most kids don't understand that the CU is not intended to be a swing and miss pitch in most cases. Throw it in a FB count and get a three hopper to SS, that's great pitching. Lots of kids never learn to trust their CU becasue they are trying to do to much and miss bats. Grip it like a CU and throw a FB down, or down out of the zone.