From what I understand of satellites and rovers, the only parts which are totally critical to get right are the modem, power to the modem, and bootloader. Almost everything else can be adjusted if a fault occurs (and they often are).
That works if you have a low data rate, last ditch omnidirectional antenna fall back. Otherwise for deep space satellites you've got to run quite a bit of stuff to get and perhaps keep the antenna pointed at a transmitter, which for a satellite includes station keeping functionality.