In the World that I work in … sending ET out on its own and expecting it to do the mission package you programmed it to do with the run-time life that you have built it for and being able to get the data back to the home craft by either returning or a communications with the launching platform or others before it goes to the bottom of the ocean somewhere.
So the big show killers are Navigation, Power, Sensors and Communications. Those all are expensive hurdles to jump. If I had to rank life, I would put Power at #1 because nothing else works without it and then very accurate Navigation or homing capability so you don’t loose a platform.
Communications next because without that you might as well leave it on the kitchen table and come back after a while and stare at it. Same results without having any data from the platform and last but not least, depending on what you want to do, is the Sensor suite.