Whatever reward system we pick, it has to fit what we are trying to teach. And there needs to be a level of consistency, so that children know what to expect. Rewards can destroy motivation when they become the only reason that children work hard at home or do well in school. The danger is that when the reward stops, the child's motivation stops. That's when the reward does not fit the behavior we hope to teach.
Rewards can have a negative impact when they are overdone.
I get concerned about reward systems that, for example, pay children every time they read a book. It worries me the message gets conveyed that we read only when we get paid. I worry about the message we send when we go on automatic this way. Too many gifts along the way can detract from the big reward of meeting the goal. Children deserve encouragement and praise, but we need to help children stretch out their ability to keep going.