miele, I really do think that this is a tricky topic to get reliable answers because of the so many factors that come into play for shimmy wobble issues.
From my experience, racks can be a part, slight or not so slight weight placement issues, headset issues, and most importantly, specific frame characteristics that affect how the other factors come together.

re materials, steel vs alu, I think its more frame design, although I'll be honest and my two alu bikes are stiffer than my steel frames and this seems to be a factor iin them being rather stable.
My old 90s alum Spec Rockhopper , even with a front suspension fork, has a typical of that period very solid alum frrame and thick chainstays, kinda like Cannondale tourers. Ive ridden this bike with a heavy rear only load and the inherent stiff frame handles it really well, so I do think the frame characteristic is a big part with this bike.

My Troll can have a slight wobble shimmy if I have too much weight in my rackpack, on top of my rear panniers, so I learned very quickly to keep the high up weight to a minimum, and it immediately went away (I keep my heavy extra water or food in a pannier now, and have tent and other less heavy stuff up high in rack pack)

Ive always noticed that being reasonably even loading side to side weight wise helps also, with front racks also, in an overall sense, so I tend to instinctively try to balance out pannier sides for weights. That said, both my old mtb and my old steel commuter ride really rather well with one heavily loaded pannier only with groceries--ie one stupidly heavily pannier on the bike and thats all--so clearly diff frames can handle weight differently.

My troll is my most robust bike, and I figure the very sturdy and wide front fork plays a part also, as well of course as the Tubus racks which are really solid. To add to the racks issue/strength, I have fattened up my rack rails so that my panniers fit on them tightly, ie there is no play at all or looseness, so panniers are solid , dont sway or even budge at all which Im sure helps with handling as everything is one good solid mass. In the past, Ive had panniers that swayed a bit, and were loosey goosey a bit all in all.