In episode 29 an amanto sword collector who is looking for the best sword in the universe tries to steal Gintoki's sword.

After hearing Kagura broke a bridge with this sword (which she stole from Gintoki), the amanto (who is supposed to be harder than steal and inmune to bullets) attacks Kagura and Kagura blocks the attack with Gintoki's sword. 
Then the amanto claims Gintoki's sword is the "Youtou Hoshikudaki" a special sword he's been looking for. 
Then Shinpachi tells Gintoki the amanto is looking for "the strongest sword in the universe".
Then Shinpachi says to Gintoki that his sword might be the objective of the amanto, since it's stronger than other swords and can break anything. 
Then the amanto claims he has seen may swords, but Gintoki's sword is unique. 
Then we see a tv commercial showing swords like Gintoki's on sale, they are indeed called "Youtou Hoshikudaki" and are claimed to be able to destroy "rocks, meteorites and muscles" and then...

Kagura breaks the sword. Sure Kagura is very strong because she's a Yato. But later in the series we see the sword being broken several times. Once the sword is broken in the fight against the benizakura, and Gintoki's given a stronger sword by Tetsuko to be able to fight the benizakura's wielder. 
And then Gintoki doesnt seem to be using this special " stronger than the Youtou Hoshikudaki" anymore, and in his battle with Jirochou his wooden sword is broken again and he loses, and in the rematch with Jirochou he wins using another apparently ordinary steal sword. 

Is Gintoki's wooden sword one of the strongest swords, or is Gintoki's strenght (or Kagura's when she used it) what makes it seems to be strong?