Swashplate compared to fixed-blade system
Makes no difference. The swashplate is there for controlability, not efficiency.
Of course single-rotor fixed-blade system can't be used, because it wouldn't be controllable, and with four or more rotors nobody uses swashplate, because it is controllable without it, but that's orthogonal to efficiency.
Single-rotor compared to multi-rotor
The efficiency depends on area for weight and RPM. Increasing the area reduces the induced power (energy carried away by the slipstream), but it also increases the parasite power (energy needed to overcome parasite drag of the larger, faster moving blades), so there is some optimal size for given lift.
Now the thing is that as weight increases, the optimal area increases slightly slower, and the best possible efficiency decreases. From this it follows that four rotors of quarter the lift each are more efficient than one big rotor.
This is however offset by the weight of the structure and transmission needed to use four rotors, and by larger turbine engines being more efficient. That, plus all the safety benefits of a swashplate system is why single-rotor helicopters are more efficient in practice.
For small sizes the optimization criteria. Due to the square-cube law small scale UAVs have plenty of power for their need, so the efficiency is not that important, the materials are plenty strong for the need, and the simplicity of fixed blades driven by four separate electric engines is the main reason to choose multi-rotor setup.
Other reasons for swashplate
- Swashplate systems are capable of efficient auto-rotation, so even in case of an engine failure, a helicopter can still glide to a safe(ish) landing. This requires reducing the blade pitch, so fixed-blade systems can't do it.
- Swashplate can compensate for the different amount of lift on the advancing and retreating side by adjusting the blade pitch, so it copes better with high flight speeds.
- Larger rotors have significantly larger moment of inertia (moment of inertia grows with fifth power of diameter!), so their RPM can't change quickly enough to give practical control response and variable blade pitch is needed to control their lift instead.
- Multi-rotor with separate engine for each rotor have a problem dealing with engine failures. If an engine fails, they have to reduce power on the opposite side by the same margin and still not only have enough power to stay aloft, but also enough spinning rotors to stay controllable. That's why the Volocopter has so many rotors (rather than just four).
- Electric propulsion does not provide the power-to-weight ratio needed for large helicopters, but separate electric motors is what makes multi-rotors simple.