There's some disagreement whether Comfrey should be taken internally, and I know many people have taken it internally for years. It does contain allantoin, which is good for you. It also contains pyrollizidine alkaloids which are very bad for your liver.
I'm definitely not as well versed in herbal medicine as you guys, but I do know about pyrollizidine alkaloids and livers. In low doses, the liver can handle it. But repeated doses over the course of time could add up.