The biggest hazard is launching the payload, if it fails it falls out over a large area causing contamination of the nuclear fuel. The high orbit of the test vehicle lowers the risks for the other outcomes you identified, and they are planned to remain in these so called "disposal orbits" for many hundreds of years. Things can get very very far apart in space. The Russian recon satellites were operated in low earth orbit and their failures were well documented and even attempted to mitigate by the soviets, though they did fail with very bad consequences at least three times.