The main disadvantage of Jello is that it contains additives and preservatives, which can be unhealthy if consumed in large amounts. Additionally, some people may find that the texture or sweetness of Jello is unappealing. It is also high in sugar and carbohydrates, which can lead to weight gain if consumed in large amounts. Lastly, Jello does not provide any nutritional value and is often considered an empty-calorie food.