My definition of iffy science is when something is isolated, and then broad speculations are derived from it that aren't logical conclusions. I believe the gender science that has emerged in recent years is one such example. I base this on personal observations on who is utilizing the assertion, and the context they are using it. Loosely falling into this observation will be an extreme majorities are forced into that are not healthy.
At my previous place of employment, a young woman asserted that she identified as a male. She was extremely shapely, Wore extremely feminine clothes showing off her figure. Her genetic makeup evidently not getting the message from her chromosomes (using the science you quote) that she was actually a male. She was allowed to use the men's restroom, which was the reason for her proclamation.
No man working there was comfortable for this. And given the current political climate of Metoo/believe her where the man is considered guilty until proven innocent this uncomfortable feeling is magnified into a potentially dangerous situation for any man finding themselves in that restroom with her alone, especially if their genitalia is out as is often the case there.
Another example of this was when my partner addressed on of her clients children as a boy (he clearly was) and the mother reprimanded her recognizing the physical attributes. In this case the boy who evidently identifies as a female was dressed as well as a normal boy. My partner was later written up by her employer for concluding the boy was a boy and not somehow intuiting that the boy identified as a girl. Dangerous stuff, can cause loss of job and possible imprisonment. In the case of the female at my previous employer, I guarantee that if she decided she was offended by any of the biological males in that room, Human Resources as well as the judicial system would suddenly offer her all of the protections afforded a female.
It really comes across as a manipulation by those I've encountered using this "science" to justify their actions that they demand accommodation for. None of which sets well with me. I know what I see, I know what the possible ramifications are and it doesn't add up to anything positive from my vantage point.