I know this is hard to answer, maybe even wrongly asked, but the point is to try to enlist and rank among all UAV components, which is more and less likely to fail.
for a hexacopter:
1. frame structural stability
2. lipo batteries
4.1. CPU and firmware
5.Radio receiver and telemetry
8. connectors and cables
Some of these are critical for flying, others are not but affect the mission.
Calculating the probablilty of failure and the MTBF or MTTF is difficult and complicated, but this forum has lots of data and experience that could be harnessed to obtain some trends.
so, what is in your opinion the part that may fail first or is more fragile?
is there a way to use redundancy? im willing to use two or three autopilots if necessary (plus a supervisor computer). I had a few unexplained crashes and all I can conclude is that it has to do with a slightly larger propeller than the motor can handle that leads to bad sync with ESC.. only sometimes. Solving this would requiere individual current and RPM sensors for each motor (or a smaller prop, maybe a new ESC just in case).