The blades rain down. Caster gets hit by them half way through the game, and Archer does it quite a few times. But UBW is the Reality Marble. UBW isn't a sword raining down attack. Archer just projects a bunch of swords and makes them rain down on his opponent. It has little to do with the reality marble.
And the reason that only Shirou can beat Gilgamesh is because if he uses his reality marble, hes basically using the same thing that Gilgamesh uses, making them equals.