Staraptor has a ridiculous amount of power, which makes it so that very few things can break through it. Staraptor's speed tier isn't too terrible by UU standards either, which means it outspeeds a lot of the meta, and its power means it could literally break the entire UU tier in half. Flying STAB is also insanely good in the UU metagame and only things like Bronzong and Rhyperior help against it. For a testament to Staraptor's power, here are some calcs:
252 Atk Choice Band Staraptor Close Combat vs. 252 HP / 184+ Def Eviolite Porygon2: 204-240 (54.54 - 64.17%) -- guaranteed 2HKO
252 Atk Choice Band Reckless Staraptor Brave Bird vs. 252 HP / 252+ Def Eviolite Dusclops: 133-157 (46.83 - 55.28%) -- 69.53% chance to 2HKO
252 Atk Choice Band Staraptor Close Combat vs. 252 HP / 4 Def Registeel: 260-306 (71.42 - 84.06%) -- guaranteed 2HKO
Right here, you have yourself a true depiction of complete broken power. Even Dusclops, one of the most physically bulky Pokemon around, cannot hope to completely avoid getting 2HKO'd by Staraptor; and that's a guaranteed 2HKO after Stealth Rock, which is commonly on the field, per se. Porygon2 is also known for being pretty bulky and it gets 2HKO'd pretty handily with good defensive investment. And as I said, Flying has very good coverage in UU, and even resists like Raikou and Zapdos get hit pretty hard, while Zapdos cannot endure CB Returns. Scarf and LO are also possibilities. Staraptor's insane power and excellent coverage simply made it a broken as hell force in UU.