These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Also, when Taran fights Dorath, he uses a sword he forged himself, while Dorath fights with the sword he stole from Taran (which was given to Taran at the beginning of The Black Cauldron). Taran's new blade proves so much better that it actually shatters the old one.
Dallben gets one when Pryderi comes with a group of soldiers to kill him.
Rhun, previously identified as a well-meaning clod, pulls off a beautiful one in The High King when he confuses the enemy into thinking he's brought a giant army with him.
Fridge Logic: Okay. Dyrnwyn can kill the Cauldron-Born. Why did Gwydion not twig onto this while he had it in his possession?
Taran tried using it on the Horned King. The sword was too dangerous to use at the time.
Also, how often do you try to stab things that you know can't be harmed? Bear in mind that they hadn't read the full version of Dyrnwyn's runes yet.
Most importantly, the Cauldron-Born haven't been around since the second book since Arawn has been holding them in reserve now that he has a limited supply of them. The fact that they get weaker and slower the longer/further they are from Annuvin also makes them far more effective as guards than an assault force.
And note that when Arawn is ready to use them, the first thing he does is steal the blade. And after using them, he immediately has them head for home.
Nightmare Fuel: Reading the final book, it seems that at least part of why actual displays of full-force magic are so rare in the series is that Prydain-style spellcasting results in this when fully unleashed.
Tear Jerker: A half dozen in the last book, easily. A very brief list - Coll's death far from home in a barren wasteland, Prince Rhun's death, Fflewdur Fflam destroys his beloved harp and has to leave Prydain forever, Doli and the Fair Folk leave the surface world, Gurgi has to go back to the Summer Country with the Sons of Ddon...