Sorry, but extreme damage? No. Nisi required very minimal healing - one targeted Cure/Physick/Adlo + Regens was enough to keep the person alive. Never, with any co-healer, have I had to discuss with them how to handle Nisi. It's always spot heal, or 'I'll throw a Cure on top one, you on bottom' and WHM/AST regening them.
Even if the group decided to do the 'legitimate' strat, which is overall frowned upon because 'over complex, why do it when we can just sac and not deal with Nisi entirely?', the healers would only need to do the above healing method each time passed. It was not extreme in any way, shape, or form in comparison to other damage mechanics in Gordias/Midas.
'Extreme damage' is more like preys in A8S, Bio in A6S and A8S, the death Sizzlebeam in A7S, Cascades in A3S... Nisi is a semi-potent DoT that gets less potent the longer its on. It was nothing to deal with.