More specifically, many fans are paying well over the odds to get the normal One Ring card because it's so powerful
If the ability that's copied has an X whose value was determined as it was activated, the copy will have the same value of X
For Hearthhull, I think it comes down to the fact that this deck is dumping tons of lands into the graveyard and it most likely wins the game by bringing them all back in one go - using a card like Splendid Reclamation - for a mass of triggers
The 10 Mega Pokemon featured are Charizard X, Gengar, Dragonite, Lucario, Gardevoir, Froslass, Diancie, Eelektross, Hawlucha, and Scrafty
It doesn't have any other effects, but I wonder if it can't be helped due to the timing of its appearance