I find you have to get really good at knowing how to harass early game with both Sona and Soraka.
Mind and late game you need to be positioned very well. Also, a mistake I see many sona and soraka's do is run away from team fights when they are targeted. Never run from your team. Move towards or around your team. That way they can A: Kill them or B: Make them switch targets. Remember, you can heal a lot. So if placed well, you are a pain in the behind.

Both Champions can be very good if played well.
Most players who have never used Sona or Soraka claim they suck when they do try to use them.
They don't suck. They just haven’t learned how to play them properly yet.
Many think all there is to Sona is spamming qwer. Sadly there is more to it.
Unless its late game and you have a lot of mana/cdr. Then you spam like a monster.

I do agree though. Sona really needs a buff. Not much, but something to make her late game better.

(I so just posted on my brothers account XD)