Druids are toughest to heal with because the only spammable heal they have is Healing Touch and this is a long heal which isn't nearly as mana efficient as Greater Heal ro Healing Wave. Druids are efficient healers when they use their HoTs, but keeping up with which HoTs you have applied when and making sure you don't over HoT and waste mana would be tough, even with good macros. I've found I can do a pretty good job knowing 1) when to cast my single target heal and 2) when to cast my aoe (proayer of healing or chain heal) heal, but that's all the bandwith I have for my healers when running instances.