One thing that comes into play when it comes to the various shimmys folks blame completely on the load is the rider themself. There is phenomenon called pilot-induced oscillations in aircraft and the effect also occurs with bicycles. The shimmy that folks talk about in bikes is the same thing. I can't help but notice that among the riders I know there are some that never seem to have a problem with this no matter the setup and some that seem to have a problem with every bike they own at some point.

I always intuitively felt that I could avoid the shimmys or suppress them when they started but didn't fully get why until a pilot friend observed that they were PIOs and likely have a large rider component in their cause.

The wikipedia page on PIOs says "Pilot-induced oscillations may be the fault of the aircraft, the pilot, or both. It is a common problem for inexperienced pilots, and especially student pilots, although it was also a problem for the top research test pilots on the NASA lifting body program". Knowing that there is a pilot responsible portion of the cause may allow some to short circuit the feedback loop that causes the shimmy.

I am not suggesting that folks not try to use a setup that is reasonably balanced and carries the load sensibly, but understanding that there is a rider component and how that happens might help avoid it. Reading a little about how PIOs occur may help avoid them.