For just 1 blue mana, it counters a noncreature spell unless your opponent pays 2, which is often a back-breaking tempo swing in the early game
If at least one creature in combat has first strike or double strike, you can activate Yuffie's ninjutsu ability during the first-strike combat damage step
It's also close to a one-sided wipe in a very popular archetype, though obviously if any of your foes bring +1/+1 counters too, its strength goes down significantly
Leading the MTG Fallout deck is The Wise Mothman, an insect mutant that creates Rad counters while also benefiting from the counters Milling nonland cards
It defines Modern right out of the gate while giving Commander and Legacy players new toys and valuable reprints
Green Slime, on the other hand, is a way to not only get rid of powerful pieces like Rhystic Study or Smothering Tithe, but also stop them triggering at all