They are avoiding giving us the exact rarity, as they did with The One Ring, because that data point unlocks a whole lot of additional information for us: number of packs, how many copies of a card exist, things like that
You need to operate at sorcery speed for this to be effective
Youll still want a nice bell curve of mana costs in your cards, but unless youre playing cEDH (competitive EDH, a hellscape where games end before youve even finished shuffling - fun, but in a horribly different way), you can have a bit more fun
Samut takes full advantage of this, granting other creatures the player controls +X/+0, where X is their current speed, as well as making non-creature spells cost X less to cast, where X is their current speed