Many believe this...simply isn’t true.Some believe compressed loads can break down the extruded kernels "possibly" affecting ES and SD. I just don't like it for personal reasons.
There is no way powder kernels can crush other powder kernels unless extreme pressure were used. More kernels get damaged by the powder measure drum while charging than from a compressed load.
I try to avoid them simply because the bullet can back out after seating...have witnessed this using Retumbo in a 300WM with 200gr AB’s and a short OAL that lead to a .050” back out over night, on average.
Had to increase neck tension AND reduce the load to get it stable, in the end it wasn’t worth the trouble, couldn’t get those ragged one hole groups again with the tweaked cartridge, so I abandoned the whole episode.
If the powder fits now, I will use slightly compressed loads, but if it’s too much compression I will try a different powder that takes up less room.