She is really reliable when she gets in range to do her combo. She's balanced because getting in range is pretty hard.
She has lower range than just about every mage in the entire game, has no mobility, and is pretty squishy. She can use her E to make her slightly tankier, but it doesn't really help that much.
She also does not have her crowd control up 100% of the time, as it is based entirely on her passive, which isn't always up.
She compensates for these weaknesses through reliability and damage. She has some really high burst on really reliable abilities that are gated because, again, she has poor range, poor mobility, and is squishy.
Of course, you can argue that Flash + Tibbers + the rest of her burst alleviates her range and mobility problems, but that is more of a problem with Flash being way too strong at removing a lot of champion's counterplay and not a problem with Annie not having weaknesses.