The effect of the Great Rings upon Men

[Belegorn]

I don't know if the Witch-king had a sense for Dúnedain which both Boromirs were. The Boromir I had mentioned was the Steward Boromir who got stabbed by a Nazgûl when he drove them off in Ithilien. On the same issue with Dúnedain and the Witch-king he did fight them for hundreds of years in the North in Arnor and eventually destroyed Arthedain. There was also Eärnur who was clearly a mighty Dúnadan and there is no implication that the Witch-king feared him at all.

"Eärnur was a man like his father in valour [Eärnil II led Gondor's armies against the Wainriders and saved Gondor from destuction. He also had defended Gondor's southern borders], but not in wisdom. He was a man of strong body and hot mood; but he would take no wife, for his only pleasure was in fighting, or in the exercise of arms. His prowess was such that none in Gondor could stand against him in those weapon-sports in which he delighted, seeming rather a champion than a captain or king, and retaining his vigour and skill to a later age than was then usual." [Appendix A: Gondor and the Heirs of Anárion]

Even with all his prowess the Witch-king attacked Eärnur when his army was wiped out.

Originally Posted by Appendix A: Gondor and the Heirs of Anárion

he singled out the Captain of Gondor for the fullness of his hatred, and with a terrible cry he rode straight for him. Eärnur would have withstood him; but his horse could not endure the onset, and it swerved and bore him far away before he could master it.

This is the time when Glorfindel said he wouldn't fall by the hand of man. The Witch-king taunted Eärnur ever since until he finally could not be restrained by any of his men and went to duel him years later to his own end. I do not quite know why Boromir himself put the fear of the Valar into the Witch-king for I do not think he was the greatest Dúnadan he had ever come across. It seems most or all of the Stewards were mighty, even a shrunken Denethor ll, and clearly the kings were great warriors too.