I don't think I always understand NTs, nor do I always understand NFs or any other type. But in the case of NTs, there is a certain directness and logic that is more common than among most other types, so if you can follow their logic, you can often see what is motivating them and (in the case of conversation) you can pick up on vocal tone to get an even better idea of what parts of an argement are important to them. The same could be said of any other type, but since NTs (and some STs) are more logical, it applies more to them than those that are opperating on feeling (or an emotional logic).
It may be more accuracte to say I appear to undestand the facade that a lot of NTs use, but it may be a false understanding if there are undetectable emotions at work underneath the impersonal logic. I don't think its easy to emotioanly read an NT, but it may be easier to notice in a more gneric sense when something is "not right" with one and to followup and see if they are willing to talk about it (and if there is resistance to talking about it, then I'm not any better at coaxing them to open up or understand them than anyone else).
In the case of INTJs, Fi is your tertiery function, so you actualy have a lot more in common with NFPs that is operating at a subconscious level that you may or may not be aware of. To a lesser extent there is also a share Te bond in NFPs and INTJs.
NTPs and NFJs share Fe and Ti, but I don't know if they notice any shared commanality because of it or not (thats somethign for NTPs and NFJs to comment on )