The least compelling thing about the Smurfette Principle is that it is hard to prove in an objective way and is easily manipulated to make it seem true at all times.
The Smurfette Principle, as first put forward by Katha Pollitt in 1991, held that the vast majority of stories aimed at children (in movies, on TV, and even in books) are male-centric. Female characters never have much in the way of distinctive personalities and are not very important. They tend to exist only as foils to the men or helpers or in some other role that is centered around the man. Since 1991, other writers have expanded the idea to include stories told for adults as well.
What is least compelling about this principle is that it is so hard to prove objectively. It is true that we could take some given set of shows and books (perhaps hits and best sellers) and count up the male and female characters. But the heart of the Smurfette Principle is more qualitative. It argues that female characters simply are not as important in the stories as male characters are. This is much harder to prove because a person who wants to make a point can interpret characters any way they like. For example, a person could look at the hit show Game of Thrones and define the female characters in different ways. Are they just pawns of the male characters or are they (Arya Stark and Cersei Lannister, for example) truly important characters in their own right?
Theories like this are based so much on the way in which observers interpret various characters. This subjectivity is the least compelling aspect of the Smurfette Principle.