We had an Irish setter with one undescended testicle. They can actually strangulate if they aren't removed and are prone to cancer, I believe. It's a more complicated (and expensive) surgery.
Hopefully Lady M and JMM will jump in here. They will know much much more.
Here's a little more information:
Undescended testicles have an increased tendency to grow tumors over descended testicles. They may also twist on their stalks and cause life-threatening inflammation. For these reasons, neutering is recommended for dogs with undescended testicles. This procedure is more complicated than a routine neuter; the missing testicle can be under the skin along the path it should have descended to the scrotum or it may be inside the abdomen. Some exploration may be needed to find it thus there is often an incision for each testicle. The retained testicle is sterile and under-developed. If there is one descended testicle, this one will be fertile, but since retaining a testicle is a hereditary trait it is important that the male dog not be bred before he is neutered.
