In order to get good solid scientific information on some dangerous procedure that has no good reason to even be attempted, someone has to go ahead and do that, many times, under all possible conditions, for no good reason whatsoever.
Be my guest!
Lol... There are good reasons to do it if you've ever gotten some not-so-clean surplus that's been around the world a time or two and in warehouses that aren't all that sanitary.
Like I said, if you have no scientific evidence to support that tumbling harms the powder or changes the charateristics then you're just repeating old wives tales.
Scientific evidence should be easy to get if it's as dangerous as some think. It wouldn't take much testing and saying testing in "all possible conditions" is a red herring that can't be done in any scientific process.
I'm still waiting for some verifiable evidence that even one shell was detonated by tumbling.