A lot of plastics are chlorinated, not just pvc, and when you burn them, you release hydrogen chloride into the air, which instantly recombines with the water to form muriatic acid.
In addition, flame retardants, most commonly bromated compounds, reduce the heat of combustion enough to cause the low combustion issue.
I don't have any information about heavy metal use, but most plastics are a mix of chemicals, and if you don't like unburnt hydrocarbons, don't burn plastic.