A couple of points:
The fluid in a Fluidamper isn't a fluid, it's a gel.
The damper is for torsional vibrations, it has nothing to do with the balance of the motor.
Elastic type dampers will work best at a certain frequency of vibration and the effect will diminish the farther you get from that number. I expect that the viscous (fluid) type dampers will be effective over a wider range of engine speeds.