I did a Wisdom Panel test on my dog and got a highly-improbable result back. The company seems to have pretty good customer service these days, so I sent them pictures of my 40-lb dog and his 35-lb mother and asked them to please explain how she could possibly be half Bullmastiff. An actual geneticist called me and geeked out about dog breed genetics with me for way longer than I expected, and I learned a lot about how the test works.
Basically, their DNA information is based solely on registered purebred dogs. Other registered purebred dogs have genetic signatures that cluster tightly with the sample members of their breed. The further a dog is from registered lines, the less their genes will cluster with those of their breed, and the more likely it is that the test would show a false positive.
The results you get for the Wisdom Panel are just a tiny bit of the information the company gets from your dog. The computer spits out the most likely or best-fitting result and that's what they send you. My dog's mother was a purebred or extremely purebred-looking working ACD from working lines, but not registered or from registered lines, so her gene result didn't cluster as tightly with the Wisdom Panel ACDs. There was also a strong signal for Bullmastiff, so the first projected breed mix the computer spit out had a false positive claiming my dog was 1/4 Bullmastiff.
The geneticist then made the assumption that my dog had one purebred ACD parent and entered that into the program. The computer spit out a few different projections including some with Xolo in the mix, but she said they get a lot of false positives for Xolo in dogs that are part terrier. The breeds that came up consistently in every projection were Bullmastiff, AmStaff, and CKCS, with my dog's sire being 1/4 each of those plus 1/4 unknown. I asked if CKCS just meant 'spaniel', but she said that CKCS was consistent in every projection and no other spaniel breeds came up in any of the results. She also said the sire was likely a mid-sized, short-haired, tri-colored dog with big floppy ears. The people I got my dog from as a pup said they thought the father was the neighbor's beagle mix, which fits that description nicely.
So I had a really great experience with the Wisdom Panel, but largely because I went a bit further and didn't just look at the initial result and dismiss the test as a novelty.