Harpie's Feather Duster is so powerful it's currently limited, as any deck has free access to a complete wipe of Spell and Trap cards
Ultimately, as always, theres no way to properly predict the future, even if we can look at past trends
SA cards appear in approximately 20% of boxes (~1 in 5 boxes for any SA, ~4% per box for Mewtwo V SA specifically)
The play pattern of playing a huge monster in a turn is also complicated, making it easier for an opponent to get a tempo advantage
We also have four total enchantments (two of which happen to also be lands in Urzas Saga and Valgavoths Lair), a couple of kindred cards, like All Is Dust and Eldritch Immunity, and are running the only valid colorless battle, Invasion of Ravnica, to get all nine legal card types in the deck