I feel that Ni-Fe watches people like it watches a river, relying only on the most underlying concepts as holding certainty. I have noticed some INFJs having a great deal of certitude, but also I have noticed that there tends to be a consistent issue with dichotomy. They need connection and openness, yet are private; they have deep insight, yet can be oblivious; so could Ni be capable of both extreme certitude or equal uncertainty? I think Ni has some propensity towards reconciling extremes that results from internalizing the biggest possible picture of reality that takes all concepts to their extreme possible end points.
Ne is much more fluid as it moves about reality and concepts.