Polyurethane has always been used as a bushing material because it is able to absorb some energy, while still being stiff enough to limit movement- thus giving us the choice to dial in the optimal density for performance vs comfort. The reason many other companies choose to use full billet inserts stems mostly from the fact that they do not have the capability to offer polyurethane, and they would rather sell a part to you than not- regardless of the fact that it is not the optimal choice for material. Additionally the full billet metal inserts will be significantly more destructive to the existing rubber in your transmission mount which will rapidly deteriorate its integrity. Over time this will open up a larger gap and will induce extra play, ultimately making the full billet mounts ineffective. This is if the full billet inserts do not bend or snap first. Ultimately polyurethane has been, and will always be the best choice for this application.
⚠️CALIFORNIA WARNING: Cancer and Reproductive Harm - www.P65Warnings.ca.gov