The tools at the disposal to Sultai players around the world are extremely powerful. Deathrite Shaman and Tarmogoyf have proven their dominance for some time now. Being able to combine those cards with the best card in Legacy (Brainstorm) and one of the most versatile removal spells in a format full of cheap permanents and cheaper countermagic (Abrupt Decay) means that Shardless BUG is going to have serious game against a wide variety of opponents.
Shardless Agent gave this deck a new dimension. Not only does it provide card advantage just through the cascade, but Ancestral Vision is already a card powerful enough to be banned in Modern. With no help, Vision is fantastic, but with Shardless Agent providing a potentially free Ancestral Recall, things get out of hand quickly. Combining Ancestral Vision and Shardless Agent with Brainstorm or Jace, the Mind Sculptor to put the Vision back on top of the deck (which, by the way, are just 2 more of the most powerful cards printed ever), this deck has incredible power and synergy.
The Shardless BUG archetype has lots of game against anything you throw at it, but the inclusion of Shardless Agent means that the deck can not play cheap counter magic that could possibly be hit off the cascade. Not having quite as much cheap interaction against combo decks outside of Force of Will (which, despite being “free” manages to dodge the cascade thanks to its “alternate casting cost” of 3UU). Against decks like Infect, midrange, and even Miracles, the cheap removal and card advantage is fantastic and the versatility of the uncounterable Abrupt Decay is a real challenge to beat.