The ammo companies do it,top/knowledgeable competitive shooters have been doing it for years.Brian Enos does it,and states so in his reloading video.
I conducted my own testing in a Dillon vibratory tumbler.I tumbled assorted rifle and pistol rounds from different lots of ammo.I had "un-tumbled" rounds as a control.
The idea is to tumble long enough to remove case lube and a final polish/cleaning.This normally takes about 15-30 minutes with corn cob.After tumbling I disassembled the rounds and found the powder had not turned in to baby powder like many have claimed.IMR stick propellant was not broken into smaller pieces.All the primers lit just fine and I did deprime some
as a sample and could not find any that had broken up or turned to dust.I have also run ammo for several hours just to find out and no problems,but I normally do not tumble more than 30 minutes.
Just use some logic:
Imagine all the billions/trillions of rounds of ammo that have been shipped by the military and ammo companies over the decades.The ammo is moved by various trucks,trains,C-130s,etc,bouncing and vibrating over all types of roads and runways.
Its seems to hold up just fine.
I routinely pull surplus ammo down and inspect it before shooting.Some of it dating to WWII.I have yet to see any with powder turned to dust or primers coming apart.Again we are not talking about ammo that has had powder/primer compromised by moisture,heat,cold or old age.
I will continue to vibratory tumble ammo(not rock tumbler/cement mixer) and call BS on the whole subject.If the ammo companies do it,they must do it for a reason.