Supertypes, such as legendary and basic, aren't card types
Mark Rosewater said it best: The main protagonists and antagonists got to be mythic rare
This means that an opponent can respond by destroying your creatures to reduce the amount of damage dealt
Sure, it's going to do a ton of damage to your opponents, but it doesn't actually do anything to the board state
Retrofitter Foundry Retrofitter Foundry provides a fine mana sink, though it performs best alongside cards like Sai, Master Thopterist that provide a consistent source of Thopters for the last ability
More than anything, though, Snapcaster Mage provides a surprise body that is ready and willing to pick up a Sword when your opponent thought the coast was clear