I've been growing Bocking 4 Russian Comfrey for a few years now. I mostly feed it to rabbits as a regular green leafy treat among other things. Also I make a compost tea (yuck!) that I feed to tomatoes in the late spring. I'm a little suspicious of all the medicinal uses... other than healing superficial cuts.
Id support many of the medicinal claims from personal experience. As a matter of fact, Im using it to treat a thumb that I dislocated earlier today.
Otherwise, youre right, it does make great fertilizer.