The problem with piece work (in an agricultural context, at least) is that is reveals a wide discrepancy in productivity between workers. The most productive workers are usually about 8x as productive as the average. Employers balk at the prospect of paying that much extra, so they create various kinds of mixed schemes that cap the worker's pay regardless of how much actual work they do. So the practical result of piece work is simply to punish below-average workers, without actually doing anything to incentivize above-average work.