There's a nice touch in the book, where the 'sword' Merry stabs the Witch-king with was particularly effective against him because it was a magical dagger which had been used by his enemies when the Witch-king was still a king in the north - just north of the barrow where the Hobbits got those swords. Distracted by that injury, he was open for Eowyn to kill him, though as you say, both stabs caused enormous injury to the stabbing parties too. Of course, the origin of the dagger was lost when Tom Bombadil was cut from the movie. Impossible to get everything into a movie (or even a trilogy of ridiculously long movies!), so it was inevitable that stuff like that was left out.

