Why are cancer screening guidelines controversial?

In general, cancer screening has saved lives. Ideally, society should focus on disease prevention. In the absence of known risk factors the next step is to detect disease early (by screening) when it is most amenable to treatment. 
The major challenge is to identify the cases that need to be treated while avoiding diagnosing patients who will not benefit from being diagnosed and who will only suffer from the stigma of being a cancer patient and the possible consequences/complications of overtreatment.