Well i main Sona and i really didnt notice that she has low damage, her Q is pretty good source of damage and its just click ability, its not like Karma´s Q that is skillshot + her passive after Q is really strong at the start of the game when landed on enemy adc. And then she has ok mid game and great late game. the most powerful thing is her W (late game mostly), when u heal ur team and shield them when grouped, idk but i don´t find Soraka´s W that good, because she can´t heal herself too. And her ultimate is just good opportunity to your teammates ults or other abilities and u can use it for escape too if u can land it properly. So overall she is one of the best supports to correct mistakes of her team, so that is maybe why she has so high winrate (Janna´s idea is the same she is just more safer support). My biggest problem most of the time is running out of mana, but that can helped by buying tear of the godess (no need for upgrading to seraphs). So maybe she is not that carry playmaking support as Thresh or Bard, but she carries by saving her team and making them able to stay more on the map and not to go back.