It isn't the material you use. It's the fact that when the engine is allowed to move in the frame it actually vibrates MORE than if it is mounted solid.
Yes, cars and motorcycles use resilient material in their engine mounts but the mounts are a totally different concept and design from your engine. They are a sandwich with no metal to metal contact between the mount fasteners and the frame. When you use rubber, or any soft material between the engine and the frame that material will compress and expand with each vibration cycle. The mount fasteners are then subjected to those vibrations and will loosen and/or fail eventually.
Soft mounting has been tried over and over and the results are always the same. More vibrations and failed fasteners and in some cases structural damage to the bike frame.
We always stress that a builder should build his bike as he sees fit. In other words, "build it your way". Nevertheless when you ask for advice or opinions you're going to get replies based on experience. That's why the majority of builders will tell you not to use resilient material in your engine mounts.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"