The assumption that a motor would be as heavy as a TGB, IGB, MGB tail pickoff and driveshaft is not one that I would make.
I'd like to see the weights. I'm imagining a pretty heavy motor, compared to the present drivetrain, plus the need for either a heavy generator, or batteries, and some heavy wire. By placing the heavy motor at the end of the tailboom, the tailboom will have to be made much more strong, thus heavy itself, and that will create an undesirable mass distribution for the whole fuselage. The available torque of a motor to drive an effective tail rotor would have to be immense to accelerate the tail rotor RPM commensurate with the pilot's possible sudden application of lots of pedal. For aircraft which carry batteries as a power source to replace fuel, a disadvantage is that when the batteries become discharged, they don't weigh any less. The component and control for electric motors would have to satisfy a evaluation of reliability to the standards of 27/29.1301 and 1309, which is daunting.
I am a helicopter pilot, and have undertaken design studies for an electric powered Cessna 172 STC (program may continue) and an electric powered R22 (program will not continue). Electric power in aviation has a bright future, in a rather narrow band of application, which I opine does not include helicopters in the foreseeable future.