It's good in multiplayer as you can use and reuse it, and outside of multiplayer it's hard to break as you can drop a land each turn and then sacrifice it for the buyback of Constant Mists
Lastly, cards like Bident of Thassa trigger from combat damage to players, and a flyer deck has a good shot of getting something through unblocked, even if its small
It is an X-cost spell that allows you to deal damage to creatures and, more importantly, creates up to three Mutagen tokens if you have three opponents
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
Thats especially useful with low-cost utility creatures like Selfless Spirit or Esper Sentinel
Along those lines, Blackblade Reforged might also be a nice add