I don’t care for the Nightsisters. I didn’t care for them pre-TCW, I don’t care for them TCW-onward. They’ve made too many appearances – been referenced too many times – in the EU that they quickly wore out their welcome for me.
I’ve only ever seen them in TCW, but I agree. The whole “witches” and “magic potions” thing feels out of place in the SW universe, IMO. I suppose it’s just the force, I just take issue with the way it’s presented.