Comfrey is another of nature's wonders that does indeed share the same stigma based on faulty and incomplete science as does chaparral and any number of natural herbs that have been proven safe and effective for hundreds, if not thousands, or years.
I personally use it, and never give a second thought to any possible dangers. Like any effective herb or natural alternative, cleansing and protecting the liver is important to help guard against any toxins released more from the success of the natural alternative than from any actual toxicity it contains.