The healing skills feels pretty game changing every time i get it. I think I'd remove it as a drop and instead offer it as the world 1 quest reward. Would make it less overpowered to stack them, but also give it more consistency. I never reached world 4 yet, so i don't know how valid my thoughts on balancing are atm.
Healing is definitely the strongest of the ability cards, but we try and compensate by making the charge time really very slow. A seed with a whole bunch of them is definitely helpful, but I'm not convinced that it's completely dominant.