A good rescue will only rehome piggies once they are fit and healthy, have been health checked by a vet and are safe to move to a new home. This is a far better guarantee than many pet shops or breeders make which is why we encourage people to consider rescue piggys first.
Some may have come from situations where they have experienced neglect or abuse in the past but the rescues will make sure they are rehabilitated and ready to go to new homes before they are adopted. Many will also do home- checks to make sure a new family will suit the temperament and needs of a piggy too.
Occasionally a rescue may look for a very special home for a guniea pig that has a long term health condition or special need but those are a very small number and will go into very carefully chosen, experienced homes who are aware of their condition and needs and can mange that appropriately.