A nuclear reactor may need to be constantly monitored, but a nuclear battery can be ignored for decades. A nuclear reactor starts a nuclear chain reaction, and then controls it. A nuclear battery is just a bit of decaying material. It will decay at its rate for years, or even thousands of years.
The Soviets used them to power navigational beacons throughout Siberia. They could guide planes with these beacons that they placed in the middle of nowhere, and did not need to maintain for decades.
If there was a car accident, it would scatter radiation everywhere. There would need to be good shielding to protect the humans around it. The radioactive material could be stolen to make a smart bomb. Because it can not be increased or decreased, it would be difficult to increase or decrease the speed. What would we do with the old batteries?
Are you thinking of Venus? Mars has one of the less corrosive atmospheres in the Solar System. It has much less corrosive oxygen than Earth.
The major problem with Mars is its low magnetic field, and light atmosphere does little to protect it from solar radiation, but that is an electronic problem, not a mechanical problem.
Kafka is completely right.