As a matter of fact their web site features couple of examples where the ZDZ owners are using these type of devices. I used soft mounting devices with my 4 strokes glow engines with a lot of success, they certainly help to attenuate the induced vibration to the air frame. The range of frequencies absorbed depends on the elastomer device to be used. Some frequencies will be not properly attenuated (even amplified if that happens to match a resonance frequncy). I think all the manufactures make its best to produce a properly balanced engine. Nevertheless, no matter how good your propeller is balanced, you will have to deal with some fair mount of vibration.
I am flying a BME 44 and is hard mounted. It was my first GAS engine project, therefore I didn't want to explore this field an used the most orthodox hard mounting method.
With this project I'd like to incorporate previous experience and implement some of the ideas exposed in the ZDZ website.