The problem with poly or urethane or any stiff bushing. Especially when you have all eight is the bind it creates.

As the rear axle articulates. No longer parallel with the bumper. The arms want to tilt with the rear axle. On a factory rubber bushing. This easily allows the arm to tilt.
With a poly bushing. The resist twisting. The solid steel arm also resists twisting.

So you force the axle to articulate. Something has to give. You replace everything with pieces that do not want to twist. The next on the list is the torqueboxes. These are the sheetmetal boxes that connect the control arms to the body. Often broken from articulation.