I have to admit this is something I wonder about. There are so many INFJs, in particular, who seem quite certain about their insights into others. I mean, I've heard other types say stuff like this too, but the "I have never been wrong about anyone, ever" thing seems to be very INFJ.
I tend to think that "I have never been wrong about anyone" is a necessarily unbalanced viewpoint and is more indicative of an inflated ego or over-confidence, than it's indicative of N vs S. However, even people I would consider more mature and balanced Ns often seem to say "when I feel a certain way, I'm never wrong about people's motives - even if it only happens sometimes", etc.
I just don't relate to this a great deal. My personal opinion is that I am indeed an Ni user, but that I am also a 6 enneagram and that this makes me question myself and my insights all the time. I don't have enormous self-confidence about my insights into people. One of the things I am working on, actually, is trusting my gut instincts more readily. Also, hanging onto my first impressions of people, and paying more attention to those, instead of letting them get swept away on a tide of warm feelings when I get to know someone better. I am finding more and more that those very first impressions are usually accurate. But I doubt they are 100% accurate. And I think that both N and S types can have very accurate gut feelings. I also have the problem sometimes that I get very strong feelings about someone or something but I can't quite put my finger on WHAT those feelings are, or to what conclusion I should follow them.
The area where I use Ni very freely (more consciously than I do in everyday life, perhaps) and am highly confident about it is in the aesthetic/cultural areas where I have some expertise. I've had a lot of confirmation in those areas, both from within myself and from others, that my insights, patterning and connection-drawing are unusually insightful and accurate (for what's that worth, as far as you can say "accurate" in the area of the arts.) But that's something different, to me. Other human beings are such wild cards. You can discern patterns of behaviour and trends and develop more understanding of how others act, or will act, based on those. But there seems to be an N tendency to think that Ns understand others better than others understand themselves. Sometimes true, perhaps...but nebulous at best.
So does all this make me more of an S? Or am I just an SJ-like INFJ who doesn't take her own instincts too seriously, perhaps not seriously enough, sometimes...which is what I suspect.