Disadvantages of ATM:
When fixed-length cells are used, the amount of header information along with the data relatively larger one is difficult to accommodate in one cell. The link bandwidth percentage decreases as it is used to carry the data. So, the performance of a device depends on the cell size. If the size is more than required, the bandwidth gets wasted and there becomes a necessity to pad the transmitted data.
This header-to-payload ratio, partially filled cells led to the inefficiency of Asynchronous Transfer Mode (ATM).