Comfrey can be made into a plant tea. When finished it has a very strong odor. Its good for tomatoes. I skip the tea method by picking the leaves and letting them wilt for two days. Then water the soil between tomato plants,lay the leaves on the wet soil and use a cover of fresh mowed grass over the leaves. Repeat during
growing season. Also good for flowering plants. Chopped comfrey leaves can be added to a compost or leaf mould pile.
Wet and bruised comfrey leaves can be wrapped in a damp cloth and applied to sore joints, sprains and bruises.
Comfrey can live twenty years and be hard to get rid of so plant it in an out of the way spot. I have mine next to the compost pile. Let comfrey grow one year before using the leaves.
The only thing I know about nettles is to run from them.