Yes there is an increased risk of cancer in the undescended testicle. You also risk a torsion of the testicle which can be a painful problem.
Substantial is a matter of opinion. You have to weigh risks of doing surgery on a healthy pup or waiting until an older dog has a problem (if it does) and having to do urgent emergency surgery.
In evaluating the difficulty of the surgery ask the vet if the undescended testicle can be palpated near the inguinal ring. If its there then the surgery will be quick and easy as if it were a regular neuter.
You can opt to have an ultrasound to try to locate the missing testicle thus pinpointing it for rapid surgical removal instead of the vet fishing around for it mechanically.
If your pup has airway issues such as stenoric nares or soft palate elongation or collapsing trachea that might push your decision away from neutering unless you must.
You can opt to wait until the dog hits puberty (ie around 6 months) to see if the testicle drops.
My personal preference is to get pups with this sort of problem neutered before I place them in a pet home as that makes the risks all mine and I know the quality of the vet who does the work.
But as that wasn't done you need to evaluate your vet's surgical skills, the difficulty of the operation,whether you want to find a vet well experienced in working on the breed to do the job etc.
I do think in general neutered pets outlive intact ones so that may figure into your decision too.
Hope this helps you!
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