I've played a game as Soraka and I did extremely well and i only got an A. While some of my teammates did a not as good got s-.
This is the game i am talking about: http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2252814030/31980065?tab=overview
As you can see my score is 0/2/25 i have the most kill participation (25) the closest to me being my adc(22). I am also the one with the least death(2) ( equal to morg).
My healing done is actually higher than anyone's damage ( i healed 34.4k, Zigg dealt 28.1k). My healing was so good that i almost healed half of the enemy team damage dealt ( they did 76.9k I healed 44% of that)
The rank I got felt unfair, Riven with 11/3/9 223cs got an s-. She has one more death ( 50% more) she has -5 kill participation, my healing beat her damage dealt by 13k. In the end, she is still considered better than me and not only by a little but by 2 rank.
I am not mad or frustated, all I want to do is pointing out that case I had that may give insight on when the system totally fails for support