INXJs are relatively the hardest to get to open up

From a theoretical and real standpoint, INXJs are the hardest types to open themselves up.

Starting with what they have in common, Ni is sensitive to information that may destroy its pristine framework. Hence, INXJs have a reputation for being stubborn with regard to their core beliefs, because as long as it doesn't agree with Ni, it doesn't agree with them. By extension, an INXJ would be more wary about the information they disclose, lest they be perceived as irrational or even crazy. This is always the case, INXJs never open up easily.

For INTJs, their proneness to working alone and sticking to what their will tells them will naturally predispose them to being solitary, and hence, averse to opening up. What makes them harder to open up compared to ISTJs/INTPs/ISTPs is that ISTJs will sooner recognize its practical importance, whereas IXTPs will unconsciously seek it as people with inferior Fe and demon Fi.

INFJs, despite the auxiliary Fe status, rarely if ever open up about their true feelings as they are much more privy to the fact that they can harbor those feelings inside instead of burdening others with "selfish" information. Better to blend in than stand out. This contributes partly to the therapist stereotype: the sole concern over the other person and the aversion towards sharing personal details helps them excel at roles like these. What makes them harder to open up compared to ISFJs/ISFPs/INFPs is that ISFJs lack the Ni that holds them back from disclosing "unruly" personal info, whereas INFPs/ISFPs as Fi-doms naturally gravitate towards self-expression and seeking someone who understands them.