Plumbatae or martiobarbuli as named in Latin were like a short javelin/long arrow with a lead weight and arrow-like head at the end. Records mention that they were thrown like javelins but also could be launched in a sling. Heavy items that are thrown in slings can be thrown with with a high velocity and can be very deadly if used by capable hands. Looking at the "dart" head, that would go right into a person easily and have a good chance of penetrating the reinforced wood for a shield coupled with the force it was thrown with as well as the lead-mass at the base of the arrow. As you said, Lawn Darts with a thin-needle at the end were dangerous, imagine something that had quite a bit more weight to it and arrow head like this. These were sure to maim and kill and were highly proclaimed for their use in combat as stated by Vegetius in his "De Re Militari" (You can find the particular segment here where they are also called the "loaded javelin") If Vegetius mentions that Diocletian and Maximian were impressed by two legions that were trained in using these weapons, it's safe to say they were very effective.