As a reader, I prefer UF in the first person. I enjoy the perspective of being in the MC's head. UF uses so many contemporary references, that I don't immediately need an A to Z, full and complete, explanation of the novel's world. I like it when the UF novel's reality comes to light through the MC's interaction with their world, rather than a lot of exposition about the rules and characters of the novel's world. I don't remember an UF novel I've read that is in the third person. Either I've never read one, or I read it, and forgot about it.
I prefer third person, or switching between third and first POV, in my other favorite book genres -- fantasy, high fantasy, and science fiction. Third person provides a chance at a detailed explanation of the unfamiliar worlds of novels in those genres. Without a third person perspective, or a MC that talks to themselves way too much, I'd be initially lost when beginning to read fantasy, high fantasy, and science fiction.