Charlie,
In terms of simple vibration caused by out of balance you are correct. It terms of torsional vibration though the rotary is a tough customer. Part of the problem is that the e-shaft on the rotary is so stiff. I have been working with Steve Beckham from the original PowerSport. Steve told me about how a 200 HP rotary just blew up the input shaft on a dyno that was regularly used to test 600+ HP V8 piston engines. This is the torque pulses of the engine, not out-of-balance. It is likely that in that case the rotary hit a amplifying couple or harmonic frequency with the dyno causing the failure. That said Steve and Everett Hatch did a LOT of work to be sure their "stiff" model reduction drive placed all the frequencies above the normal operating range. If you hit one of those frequencies with one of the rubber couplings or the bushing-around-bolt dampers it will fail period. Most of those systems try to push the first order frequency below the operating range. That is why you will often hear the engine shut down rattle with a planetary. Typically the energy in the system is low enough to pass through that RPM without damage. Usually. But if not planned for those pulses can be a disaster.
Bill Jepson