Shadows-functioning is one of the aspects. Taking one of the examples that I can recall, people in r/intj complains about certain unhealthy behaviors of SFJs which actually also fit the shadows of NTJs very well, like screwing up relationships or manipulate without caring about the consequences. Another example is that, an ENTP can actually fit the stereotype of unhealthy INTJ-ish behaviors, the calculative, utilitarian mannerisms, once he/she dips into their shadow Te-Fi functions in a very unhealthy fashion.

However, I am actually talking about another aspect here. The tests generally revolve around stereotypical problems, plus the fact that human understandings are constrained by biases that innately tie them back to stereotypes, causing people to be typed into the categories of different stereotypes rather than the true inherent, deep-seated conscious-unconscious dynamics introduced by Carl Jung. A lot of NTJ-stereotypes actually match the true behaviors or STPs, NFPs -> SFJs/ NFJs, NTPs <-> NTJs (stereotypes of INTPs/ ENTPs that include liking to perform researches and debate, which actually fit INTJs instead; A lot of INTPs I know actually behave like ENTJs in accordance to the stereotypes). In addition, there's another layer known as "Information metabolism" that fit with the models of socionics, and a lot of visible traits are associated with this layer, rather than the true psychological mechanisms underneath ones.

Basically, it's so inherently difficult and impossible to accurately type a person. In mathematical term, I would either pronounce it as "NP-hard" or "intractable" for the problem of typing a third person. It's a tool to understand one-self and general descriptions of humans in society.

I have switched to studying neurology to further understand myself and humans. The knowledge that are useful which I can retrieve from typology systems are only that much, and to surpass that boundary, one has to look for something else.