For a champion whose main thing is to heal (including self healing as well), that item cuts her abillities in half. Imagine item that you can buy for 800g that will make Syndra's skill do 40% less damage. Or item for 800g that will make Nautilus' CC 40% shorter. How about item that you can buy for 800g that takes 40% of Malphite's armor? Or...just straight up item that makes ADCs AA 40% weaker.
Like that's how this item impact Soraka players. I get the idea behind the item, but it doesn't hit healing only - it hits champion in same way as previous QSS affected Zed, Fizz or Fiora. You basically buy an item that neutralizes what champion was meant to do and you take most of their power away from the champion.