an interesting LoTR thing is how implicitly weak the Riders are at the start of the story - Aragorn holds off five of them at Weathertop - compared to the Witch-King at the end. Grip and Fang could probably fuck this dude up.

Not to mention: but they’re traveling incognito in Fellowship, on horses. Later on they’re riding what for all intents and purposes are dragons bred for battle. In the same way a Honda Accord is less powerful than an F-22 fighter jet.

I thought they gained in power as Sauron became more powerful with the approach of the ring.

I think it's outright explicitin the text that their chief weapon is fear.

Aragorn doesn’t really hold them off. He can’t prevent them from accomplishing their tactical objective. Once the Morgul knife pierces Frodo’s shoulder they leave voluntarily, believing that will be enough.

Aren't Fang and Grip cowering at Farmer Maggot's feet at that point? Physically, sure, they could; the wraiths are never exceptionally physically strong---even the Witch-king at the Pelennor Fields seems no stronger than any other mortal.

IIRC, in the book they have very little physical power. It's the overwhelming dread and horror of their presence that affects people.

"Far off yet is his doom. and not by the hand of man shall he fall." Checks out.

This is really an issue with the films. The book doesn't have the (admittedly cool) scene of the Witch-King defeating Gandalf. In the book, Théoden is ready to face TWK, but his horse panics, falls and crushes him. Then no one else has the courage to stand against TWK, except Éowyn and Merry.

Their power level increased the nearer to Sauron they were.