Have you ever worked for someone whose reactions were unpredictable? One day they were giving positive feedback for success and the next day they were dressing you down for the same results? How did/would that make you feel? What impact would that have on your desire to achieve good results? For most of us the lack of predictability would create a reduction in motivation to succeed and show initiative.
It is always better to hold people accountable for their results in a predictable and consistent manner. As always, we recommend fair evaluation of results followed by consistent/predictable positive feedback for success and consistent/predictable redirection of actions that have led to failure.
By the way, parents, this goes for your children, too. They need to know that they can expect appropriate, consistent and predictable consequences when they succeed and when they fail.