One of the main problems is that there is there is no card that has the intention/drawback of healing your opponent (except maybe holy light, and that's debatable). Things like AMing your own creature, sundialing to purposely stop your creatures, etc. are normally what your don't do, so they can't really count as viable counters. You need cards such as a creature that starts with negative attack as an additional drawback for a powerful effect, or a powerful spell/permanent that has a drawback of healing your opponent when played. These cards would be played with the knowledge that they have these drawbacks, so they would count as viable counters to SoSa.
Edit:
Oh and technically SoP would be an example of such a card. It is a weaker effect, however, so it being the only such example makes SoSa still look powerful.