This applies to a lot of champions as well.
An extraordinary good Zed can outplay an enemy laner that is supposed to "counter" him, and there is little window for enemy laners to react if they are against a good Zed. A good LeBlanc can unleash her QWE combo and nuke someone in 0.5 seconds. A good Lee Sin can perform great escape and get a good ultimate. A lot of mages are also pretty unforgiving.
@Historical saying Soraka is broken is like saying Anivia is broken because she can unforgivably kill you with QRE combo in late game and you won't even have time to react. It's absurd.
A good Soraka is scary, but there are a lot of counter-measures against her.