My second cousin subscribed to one of those things and found he was related at the level of "second cousin" to a person who had the same last name as me, but whom he had never heard of. Turns out is was my nephew, who would be his second cousin, once removed.
Also, cops use those services to find relatives of suspects whose DNA they have, so it's not total nonsense. One could easily make genetic profiles of certain ethnic groups and match individuals with these profiles (including means and variances) with a certain amount of rigor. From what I've seen, they are pretty reliable in finding relatives. BTW, by nephew's profile said that he was Scandinavian/Danish. His father was Danish.
DNA for identification of perpetrators and relatives is science.
Using DNA to say you are 0.13% Polynesian is not.