My first tourer was a Nashbar. It suffered from terrible shimmy problems when it was fully loaded and I got going fast. I discovered through trial and error that the shimmy was lessened (but still significant) when I had the load carefully balanced side to side.

The bike had 1" round tubing throughout (except the stays, of course,) and was their XXL size, so I think it was just a really whippy bike. I think it was just a road bike with eyelets on the fork, a triple crank, and 36-spoke wheels. I don't think the geometry or tubing was designed to handle a load. It also had Blackburn racks that were strong enough, but not as solid as the Tubus set I have now.

On my current tourer, a 62cm LHT, I've never had the slightest hint of shimmy. I still try and balance my loads side-to-side. Maybe I should do a horrible job of balancing and see if there's any shimmy.