Span is a true hellhole, even for a prison world. Being extremely pleasant looking from the outside the jungles of Span match the most dense Jungles of Catachan.
There is an added hardship: Humans are totally incapable of splitting up any fauna or flora on the planet for nutrients. So aside from the areas of prison sites who vat grow and recycle their own food, a human has no chance of long term survival on Span. Other than on Deathworlds, the prisons are very cheap to maintain since general planetary parameters like atmospheric composition and temperatures are perfect to sustain life.
The world is completely off limits for any space travel aside from official prison ships. Special Facilities exists for prisoners of the Imperial Guard, the Ministrorum and even for the Inquisition.