These decks tend to focus so much on their combo versus you that if you can spend one mana and one card preventing them from doing that, your other cards will be more than enough to win the game
Evereth, Viceroy of Plunder shows that you can never truly play too much treasure in Commander, despite how it makes people feel
But, when it comes to assessing the value of these cards, it's not always about their power or which card sees the most use in player decks
Double-faced cards show transformations or key moments, like Kefka, Court Mage/Kefka, Ruler of Ruin, Sephiroth, Fabled SOLDIER/Sephiroth, One-Winged Angel, or Clive, Ifrits Dominant/Ifrit, Warden of Inferno
Art cards cant be used in gameplay, however