Some incoherent and vague observations.. (I'm tired and my mind is absent, sorry about that.)
Ulairi incompetent? Hardly.. But limited.
Nazguls are not flesh and blood, they're ghosts and ghosts have "different rules".
Shire encounters: the first happens at afternoon.
Nazgul's powers are severely diminished during daylight, blurring their senses etc. Similar to human searching someone in dark.
Second encounter takes place at night, and nazgul seems more determined. However, he is interrupted by the
elves. A bunch of
high elves, to be exact. A lone wraith is no match to them. Yes, that carefree lot having their picnic is surely capable to defend themselves. And there is the song about
Elbereth, wich seems to have an "exorcistic" effect.
Weathertop:
Amon Sul is sort of "homefield" for
Aragorn, a former major
fortress of his (lost) kingdom.
Witch-King wrecked it yes, but was also driven away later. "Homefield" gives an obvious psychological edge, plus in
Tolkien's world also a "magical" one. Then Aragorn knows how to fight wraiths (NOT like in movie, btw, imo, etc), and he has "revenge bonus" to tone down his fear (history of
Arnor, plus dead
rangers at ford of Sarn). Nazguls can be harmed by fire, not destroyed but stripped.
Hobbits wield "wraith-slaying"
knives. Elbereth is called upon. None of these things alone propably wouldn't been enough, but together they make uncomfortable battlefield for nazguls. And they already managed to inflict a wound that no one in history has survived. Nazguls are not berzerkers, they have much sense of self-preservation. In their place I would've done the same thing; retreat, wait, and strike again later.
About undercover; they were undercover up north. They were "disguised as riders in black", and most people thought they were living
men, only foreign travellers. In reality they were undead lords and sorcerors. When a ghost succesfully pretends to be alive, that's undercover for me.
Nazguls aren't called
the Nine for nothing. When there's let's say five of them present, there is just five of them. But when they gather all under their Captain, they are 'The Nine', much more than a sum of their numbers. They pulled a draw against
Gandalf at Weathertop. Through that night the wizard was in deep.. trouble. Sunrise propably saved him. And his effort of drawing some of wraiths away from Weathertop propably saved hobbits&Aragorn. And maybe the battle weared the nazguls down too.
So if one knows what to do with a nazgul, it's possible to fight them (Aragorn, Gandalf). But if one takes them as another big bad cronie, one's gonna end as a cold corpse in the dark..
Earnur and even
Boromir (the older one) made this mistake; they didn't know what their foe was, and fell. (And Earnur even after he was warned
several times. Darwin award coming..

)
Much more to say but not enough brains to write it.. Hope I had a point there somewhere; good luck finding it.