I think the design space for a powerful deck is absolutely there, making the first face-down creature spell you cast each turn effectively free and making all face-down spells cantrip massively empowers the deck
For example, if you control Muldrotha, the Gravetide ("During each of your turns, you may play a land and cast a permanent spell of each permanent type from your graveyard") and you cast a creature spell from your graveyard during your second main phase, you won't be able to cast another creature spell from your graveyard with Muldrotha's permission until after your next ending phase ends
Gruul to Be Kind Atarka, World Render | Illustration by Karl Kopinski And thats a wrap on our tour of the best Gruul cards
Flexibility, X spells, mana reliability, blah blah blah
On the other hand, if the table went on a Collective Voyage with you at the beginning of the game, they are going to be suffering rather painfully under the enchantment