There are a number of cases of liver damage from taking Comfrey. This isn’t a theoretical issue. Some people died. Some people had liver failure.
Can you use Comfrey internally responsibly for certain issues? Probably, depending on the person, the problem, the dosage, the length of time taken, the dosage form, preexisting conditions and more.
Should it be used daily for prevention or general health? The potential for harm exist. The disease it causes is hard to identify and probably under-reported. So I believe no, you shouldn’t use it daily or long term.
I have many more thoughts on Comfrey that I’ll be writing to accompany the studies. For now, it seems like access to the full text of studies on Comfrey are more important. So here you go.