When a plane is slightly nose heavy, it will become more stable and less aerobatic. The more nose heavy it gets, the more noticeable this becomes. It will also become difficult to slow down for landing - but it will still fly and be controllable.

As you move the CG back, the plane will become less stable and more aerobatic, but the problem here is that once you reach a certain point, it becomes so unstable that it is difficult or even impossible to control.

Remember, the tail feathers are there for a reason - to keep the rear of the plane in the rear!

Think of a dart. It has a weight on the front and feathers on the back. The broad, light surface of the feathers will catch air to keep the point facing the front. Even if you hold a dart by its point and throw it tail-first, it will flip around so the heavy part goes forward.

Now, remove the heavy point from the dart and make the feathers out of lead. When you throw it, the heavy tail will lead the way.

It's the same thing with an airplane. If the tail gets too heavy, the tail will want to swing out in front - Not a good thing!

So it is alway beneficial to err on the nose-heavy side. As the old expression goes, "A nose-heavy plane flies poorly, but a tail-heavy plane only flies once"