What an indicator means is that it signals that there could or could not be a problem, win rate and champion balance is correlated, but not with perfect correlation. There are a number of factors that influence a win rate of a champion, and this is why win rate cannot be any more than an indicator. Win rates are affected by number of people playing the champion, which win rate you are talking about (normals, ranked, what division of ranked, etc), and the difficulty of the champion. Then there are things like disconnects and trolls that may skew the win rate even further. This is why win rate isn't the only indicator.
A champion with a high win rate should be looked at to see what is causing that high win rate. A high win rate should be an "eyebrow raiser," meaning that the champion needs to be scrutinized. Also, I feel the need to point out that within approximately 2% on either side of 50% is considered nearly perfectly balanced; anywhere from 48% win rate to a 52% is considered to be operating in a balanced state.
I answered your Blitz comments here in your other post, and don't feel like repeating myself, so ill ignore that portion of your reply.
I think Nidalee IS fairly well balanced at the moment. There are team comps and strategies her kit works well with, and there are some large shortcomings that can mess her up as well. I'm happy with the direction Riot went with Nidalee and have no problems with her anymore.........(okay the spear spam still hurts and is annoying, but is far from the same class of toxic it used to be).
Based on win rates, I cannot say whether Amumu, Malzahar, and Heima are also problems. Looking at their kits and how they function in game, however, I could say something about each one. But my point here is that my opinion about the balance of the champions isn't based solely on win rate. But yes, I think that each of those kits is a little imbalanced; but for different reasons.