Hi Darren,
Sadly there is no greater benefit to using comfrey in any form then any other plant. It is not superior in NPK, trace elements or anything else we seek. I grew and still have 3 comfrey plants that i planned on using as a fertilizer. I even made 2 batches of stinky tea. I did use the tea and anecdotally, didn't see any difference. Just recently, I put the question of comfrey use to the Garden Professors, a highly qualified authority in the world of Horticulture. They said i was wasting my time basically.
Here is a link to the discussion on facebook.
https://www.facebook.com/groups/Gard...4113272051490/
Cheers Mark