The image below shows a spaceship that has not been launched. Its success probability is 100%. Before completion, the success probability was 0%. These are the only two values I have ever observed. Under what circumstances will the success probability be something other than 0% or 100%?
The success probability is determined by the ratio of Life Support modules to Habitation modules, counting only the modules connected by Structure, and assuming there are enough Solar panels to power all modules.
A spaceship with two connected Habitation modules, two connected Life Support modules and two Solar panels has a success probability of 100%. If one of the Life Support modules is not connected by Structure, the success probability would drop to 50% (one Life Support module to two Habitation modules).
The current version of freeciv (2.6.0) selects the type and placement of all spaceship parts automatically. Since it is quite sensible about the selection and placement, it is uncommon (but not impossible) to see a spaceship with a success probability other than 0% and 100%.