Go get your Nykthos, Shrine to Nyx
But the biggest reason it that Treasure Cruise is simply a better way to gain card advantage
Sometimes casting an Underworld Breach will amount to nothing, but more often than not, it will give you access to more cards than a piece of card draw ever could
Maybe I am being too biased because I think its design is really, really cool, but I think that it can provide a lot of flexibility to certain decks in the format
As a result, this deck is filled with mana rocks like Sol Ring, Arcane Signet, and the on-color Signets and Talismans
To take a mulligan, a player shuffles the cards in their hand back into their library, draws a new hand of cards equal to their starting hand size, then puts a number of those cards equal to the number of times that player has taken a mulligan on the bottom of their library in any order