I think something any INTP will have trouble understanding about those who make decisions with feeling is that their sense of feeling is far more complex than most any T is going to experience.
When you practice using your feelings from an early age to make decisions, then you build up "emotional experience" like experience points in a video game if you will. A certain situation reminds you of an emotionally similar situation, from which you draw experience from. They probably do it half the time without even realizing where their experience is being drawn from. By the age of adulthood, it could very well be a trained response within them, without complete recollection of all the factors affecting their current emotional calculation. Maybe they have complete recollection too? I'd be curious on this.
Feelers become emotionally mature in a way that an INTP who represses their emotions will never experience. INTP emotions I believe to be raw, simple, and intense when felt. They are uncomplicated emotions that did not have to learn the rigors of decision making, where you feel the emotional state of others to assist in reaching a conclusion. Sometime long-long ago in our childhoods, something clicked in our heads that told us, "We can use logic to solve this!" and from that day forth our feelings have mostly been left on the bench, only to be used in seldom circumstances.
The emotional maturity developed in some feeling types would be difficult for us INTP's to grasp because of the sheer complexity of the emotions being felt. Essentially at it's core, it originated from the pure emotions that us INTP's do feel. Anger, sadness, happiness, ect. yet different flavors of those, combined with an emphasis on other people's emotions (for those feeling types that are emotionally mature.) Non-emotionally mature feeling types would probably put supreme importance on their own emotions rather than those of others, and base their decisions on how they personally feel.
I think many INTP's lack emotional maturity because we do not use our feeling function. Even when we improve our emotions in adulthood, I don't think it'll ever catch up to those who use it constantly.
For an INTP to walk in the shoes of a feeler, you'd have first be able to recognize all of your emotions when you're feeling them. This is a problem as much of the time, I don't feel anything, and I'm sure many of you would agree.
Now if you pass the first step and can detect your emotions well enough to understand what their motivation is, you would then act upon those emotions, for the sake of those emotions. Yet if you do this without emotional maturity, you're going to behave like a child. Remember, the feeling types spent their childhoods doing just this, acting rashly upon their feelings and learning from the experience. If you have not done this before and start today, you're going to effectively become a child in the process.
The next step would be to learn as you go, trial and error, living by your experiences and letting them reshape you and mold your feelings into something new.
I believe this is how you would describe the path of Feeling. It's not something a normal INTP can just sit-down and comprehend, we can only rationalize it, not experience it.
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Disclaimer: I'm sure there are some emotionally mature INTP's out there who grew with a healthy outlet to express their emotional side, and therefore, have progressed beyond the point of emotional childhood.
However, feeling types are more likely to be (more) empathetic since they pick up on and transmit emotions better, and have a better understanding of them, typically.
