What drawback offsets the mass savings achieved by using highly efficient ion thruster propellant?

The mass of the required large power generation hardware (solar arrays or RTGs)

While ion thrusters achieve high specific impulse, drastically reducing the mass fraction dedicated to propellant, this saving is often counterbalanced by the mass penalty incurred by the necessary power systems. For missions near the Sun, this means deploying massive solar arrays, whose physical volume and structural mass must be accounted for in launch. For deep-space missions utilizing RTGs, the nuclear source itself, along with its supporting structure and necessary power processing units, is very heavy. In fact, for these nuclear-powered missions, the mass of the power generation system frequently represents a larger portion of the total propulsion system mass than the propellant mass itself.