How reliable should military UAVs be?

The recent decade has witnessed an emerging usage of Unmanned Aerial Vehicles (UAVs) in both military and civilian applications. Compared with manned aircraft, a much larger percentage of UAVs are reported to crash every year, due to its unmanned nature and immature technologies. The high failure rate is mainly attributed to the lack of redundancy design and insufficient reliability growth tests. It is natural to ask whether the UAVs are...