The Wirecutter section of the New York Times ran an article about laundry detergent sheets a couple of years ago. It said, “But after researching and testing laundry detergent sheets from five different companies, we found they were uniformly poor cleaners, especially compared with the liquid, powder, and pod detergents we recommend. Also, the ingredients and manufacturing processes used to make laundry sheets may not be as sustainable as the companies claim.” The article says that the products are mostly made in China so getting them here requires a lot of travel. And the sheets contain polyvinyl alcohol, the same polymer that laundry pods are wrapped in.
Plus I really doubt these products are cheaper than the liquid or powder detergents.