While it can be dealt with cleanly by a Swords to Plowshares, assuming you managed to hit a creature or planeswalker of theirs, they would have paid three cards (Swords, discarded card and creature/planeswalker) for your one (Entomb/Faithless Looting and reanimation spell offset by drawing a card)
If you discard a card with madness, it goes to exile rather than your graveyard
One of the most obvious is Lutri, the Spellchaser, which would allow any Izzet Commander or Brawl deck to include an automatic 101st card
The faster you chew up the mana dorks for extra cards and lands, the quicker you get to your giant threats
If playing in anything after the XY-On format, you had Tapu Koko as an obvious partner, and the damage could quickly add up after the spread had started