Any herb with the officinalis in the scientific name (as in symphytum officinalis = comfrey) was part of the "official" pharmacopeia of recognized medicinal herbs sold at apothecaries. This would date back to the 17 - 1800's when plants were being named in the Linnean system.
Comfrey has been a traditionally recognized healing herb internally and externally for centuries. Might there be side effects to it? sure Would I recommend taking it or any other medication long term or in big quantities without checking with some kind of expert? of course not. Have you ever listened to the list of possible negative effects up to and including death, they give in the voice over on all the pharmaceutical commercials on TV? Makes you not want to take anything again, ever!
But a cup of soothing comfrey root tea here and there is the least of my worries and I've had plenty of them (cups of the tea, not worries!