Unlimited Blade Works is the result of the concept of swords being both their
Origin and their
Elemental Affinity (due to their fusion with Avalon), and the basis of their
Projection and
Reinforcement skills. It is the definite ability obtained by someone whose life was saved by a sword, actually merged and lived with a sword, and acted as a sword all of their life.
Yes, every Shirou (that got Avalon thats it) has their own reality Marble, but on this case is pretty similar to Heavens Feel Shirou, that they are basically hijacking Archer's power instead of their own. In this case is less deadly than Archer's arm though, but when their contract is over miyuverse Shirou should lose most of his power. he MIGHT get UBW but might take years (HF Shirou could master Archer arm in about a decade, probably this Shirou could do it sooner due to his better training)