Big Gator ToolsBGRSTD2000DGNP V-Drill Guide, Standard Sizes 3/8" to 1/2"

I need to drill an 8" deep shaft for a floating shelf rod to be inserted. Can this be relied on for this drill depth?

Doesn't this metal drill guide increase the wear on the drill bits and shorten their lives?

Is the size of this drill guide a true rectangle? Also, is the height of the drill guide the same on both the smallest and largest drill guide holes?

1/2" drill bit doesn't go into the 1/2" hole. Why?5