When gold cards were first introduced, not only were many of them not particularly strong, but they had a unique restriction all their own, because these cards weren't just special
Fire beats Grass
We often have cards that improve or net you a bonus when other things are played or come into play (I hate that "play" means two different things in the same sentence)
Players will be able to find Lucario, Magnezone, Mimikyu and the two new legendary dragons - Miraidon and Koraidon - in their ex forms
As soon as you throw some non land cards into the mix you massively increase the number of cards you need in your deck to find fatal, far beyond what you want to play in cube
Not only do you blink your opponents best creature out of existence for three turns, but for the next two turns, you get a temporary reprieve from whatever other creature they might commit to the battlefield