I like the functions, there's no proof that they're "real" but they're a cluster of traits basically and seem to work for the most part. People seem to understand them. The problem is Fi and Ni mostly when it comes to mistypes. Which is probably why INFJs and INFPs are confused so much. The extraverted functions being more external and objective are much easier to grasp and recognize. Ti and Si to a lesser extent but being T and S functions they're still more concrete and grounded. This leads us to Fi and Ni. Not only are they subjective, they're also F and N functions so not grounded, they're more abstract. So what ends up happening I've found is people just start making shit up, or they project themselves onto what the function is and redefine it. I've seen Ni be described as being able to mimic every function, more than once I've heard this from different people, among other outlandish things. Also just elitism about the function in general. As for Fi it's about personal values. So the arguments I see is "Person X isn't a thinker, they just value logic and reason above all else because of their Fi values" these definitions are so subjective and broad that Ni and Fi effectively mean nothing. They could mean whatever you want them to mean.