generally speaking, if you have the CG too far back, the plane will be very pitch sensitive, and uncontrollable. if it is too far forward, you may run out of elevator on landing, and not have adequate elevator response. Most likely you'll run out of elevator on landing, and your landing speed will be higher.

it is generally best to be on the too far forward end of things than the too far back. You can always adjust the CG back if it's too far forward, but most times when you have the CG placed too far back, you seldom have the opportunity to adjust, because you've probably crashed the plane.