The number #1 issue on military electronics, is utmost reliability under high stress conditions, which include (but are not limited to) temperature extremes, vibration, brutal acceleration, humidity and condensation, resistance to chemicals, extreme electro-magnetic immunity, operation at high altitudes or high pressures............the list goes on and on.
In another company we produced a power supply for a military smart device. The required reliability was in the sub-PPM level, and we had to conduct periodic evaluations from random samples to prove that the reliability was still there.
When you think about it for a while, there was some irony in that much reliability, for a device which will be used once and only for a few moments, only to be pulverized after its mission is done.