*Screws vibrate loose
*Aluminum breaks with no warning what so ever
*Aluminum loops are many times heavier which slows the bow needlessly and also causes the string to continue moving vary foward after the arrow has left the string. So much so that the metal loop can literally pop the rest if the brace height is less than 7 inches.
*Metal loops will not twist to reduce torque on the string like a string loop does
*Metal loops are hard on releases and serving
*Metal loops are much more dificult to move up or down the string when tuning your bow
*When tuning your bow you tighten and loosen the itty bitty screws enough times and the screw heads will strip out.

There's a bunch more reasons ifI need to go on.