carljung
One problem is that the thing being measured can decide how it wants to be measured. It's easy for me to select a type from MBTI.
The claim "you have one of 16 personality types" is not falsifiable. Why does it have to be one of those types?
The things that MBTI purports to measure - E/I, S/N, F/T, J/P are just based on Carl Jung's made up theories about how the mind works.
The questions on the MBTI are supposed to correlate precisely to Jung's scales, but what if they don't? Language is awfully malleable. Do I prefer abstract or concrete? Give me a break. (Yes, I get it, abstract is N, concrete is S.) Would I rather stay home and read a book or go to the party? Can I have a book reading party at home? Can I bring a book to the party? Can I read a book about a party? What if I don't like books or parties?
The whole concept of dualism is pure philosophy.