UBR is a grindy midrange-control deck with Fable of the Mirror-Breaker, Indomitable Creativity, and big top-end like Atraxa and Valgavoth, so Spell Pierce lets us counter their key early spells like Fable or Deduce before they're able to pay the tax. We need to apply pressure quickly before their card advantage engines and removal (Fiery Impulse, Volcanic Spite) take over, so keeping the aggressive core maximally intact is important. Soul-Guide Lantern is a low-cost way to strip a card from their graveyard and slightly disrupts any Flashback or graveyard-based value they pick up. We cut Boomerang Basics and a Flow State since this matchup rewards proactive speed and protection over card selection against a deck with real countermagic and card advantage.