There is some well proven theory that giving shildren extrinsic rewards can make them view a particular behaviour as undesirable and therefore can have the opposite effect to the one you want. Child thinks " If I have to be rewarded for doing that it must be difficult/unpleasant therefore I won't choose to do it"
I also know someone whose child turned it round on her and began saying he would only do things if he got a sticker on the chart first!
I think they can be useful to help motivate a child to break long term habits but with some provisos:
You don't give a sticker every time a child does a particular thing.
You choose random good behaviour and try to surprise them with a sticker.
The child never gets a sticker if the ask for one. They should always to what you ask because that is what is expected of them.
You never say "If you do this you can have a sticker".
You never take away a sticker which has been earned. Negative behaviour now does not make earlier good behaviour less valid.
There is no time deadline so the child begins earning the next goal immediately the previous one was achieved. Otherwise they may give up for the rest of the day/week/month.
They can be successful but they have a lotof pitfalls.