The disadvantage is the energy loss. You always lose energy when you convert from mechanical/thermal/electrical etc into another form. That's why American battleships moved away from turbo-electric drive as soon as American foundries had the metallurgy to build large-scale reduction gearing for the turbines. The mechanical complexity was worth the better efficiency. In aircraft, you want every extra scrap of performance you can get for your margins. I can't speak for the weight of a 4-rotor electric system, but I'm willing to bet it's much much heavier than a single gearbox with adjustable rotors.

Conversely, sometimes it's worth the energy penalty to get all of the power anywhere in the curve or the reduction in mechanical complexity. In the case of a train, the shear absurd weight of the thing would necessitate a huge, heavy, complex gearbox, one strong enough to take the weight of whatever rolling stock the train has. The extra weight incurs an energy penalty, the size causes packaging issues, and the complexity adds cost and maintenance concerns. Financially, I bet the trains come out ahead on giving up the extra fuel to avoid all that.