I have found comfrey really hard to remove. We grew it years ago to feed to our manure producing farmed rabbits. We don't keep livestock anymore but it is very persistent in places where I would like to do something else. The tap roots go down forever. But it does have lots of good Qualities. The bees love it. Comfrey is always full of the big bumbling bees as it seems to flower forever. It holds a wet bank in place for which I am truly grateful as we had hundreds of millimeters of rain last summer. It is great for the green component of the compost pile. ( Can't use kitchen waste as we live in the woods with bears and raccoons ).
The deer love comfrey which is not a wonderful thing for my property.