Since the taste is a bit more subdued than that of sugar, most people end up consuming more than what is required. For example, if a person would usually stir in a spoonful of sugar in a cup of tea or coffee, someone not used to the taste of jaggery would end up adding more than required.
Again, it’s not a free food. Jaggery contains more or less the same number of calories as sugar does. Hence, by adding in more, you are also increasing your calorie intake. Jaggery also contains sucrose, which raises blood sugar levels and may be harmful to diabetic patients.
If you don’t go in for unadulterated, well-aged jaggery, you could be eating food with preservatives in it, which again, is not really helpful.