Heavy Ball vs
Medicine Ball

Wrong size and shape. A medicine ball feels nothing like a football in your hands. The grip, the weight distribution and the release point are all different — meaning the strength you build doesn't transfer directly into your throw-in.

It doesn't bounce. Without a bounce you can't do continuous reps. You throw it, you walk to pick it up, you throw again. That's not efficient training — it breaks your rhythm and limits the number of quality reps you can get in a session.

General vs specific strength. The principle of specificity in sports training says that strength gains are most useful when they're built in the exact movement you want to improve. Training with a football — same size, same shape, same feel — means every rep is specific to the throw-in.