Since the heavy metals that cause health problems are knowingly lead and meccury only, the ceremic ones should be even safer than the cast iron one. The chemical coating on the cast iron ban is well known problem. For the seremic one, the mercury should not be a problem of worry since it is evaporated during the making. I am not sure about the lead. But we have used china in food services for so long now.
The only issue with the ceremic ones is that when cooking Chinese food, we normally goes through an high heat first then low heat. The ceremic cooking ware may not be good for the high heating.