Sweet potatoes are a challenge for cool or short-season locations. I think I could grow some here, with a lot of TLC, and have a list of varieties to trial. For me, it is a question not only of maturity, but of where the tubers form. My soil is very heavy, and I have little patience for playing "where's Waldo" with a shovel - especially having experienced that once.
About 15 years ago, I grew a sweet potato grown for its shoots & leaves, from slips purchased in an Asian grocery. The vines grew well, and we harvested a lot of greens from them. When Autumn came, I decided to see whether the plants had developed tubers. The good news: they had... long, narrow, twisted white tubers that had a very dry consistency (similar to a white variety aptly named "Choker"). There were a lot of them too. The bad news: they were anywhere within 3' of the plants in all directions, and up to 8" deep.

To find them, I had to follow the thick roots (which were identifiable) outward from the plant, digging carefully, until I located the tuber. This wasn't harvesting - it was excavation!!! After digging only a few (and ruining that part of the garden) I gave up & left the rest to freeze. I was actually relieved that none of them survived the winter.