Complexity, yes, but for a different reason -- gyroscopic precession. Cyclic inputs for differential lift require that the actual (mechanical) blade adjustment be at an angle to the change in the lift produced. With a single rotor you can accomplish that with a simple mechanical linkage (and a bit of body english -- the angle of input precession is rarely at the "proper" 90 degrees). Counter-rotating blades mean two sets of blade pitch controls (swash plates, rods, followers and linkages, etc.) and, if you want to avoid unplanned pitch adjustments, a servo system that can moderate cyclic control input based on direction (input will be much more sensitive to fore-aft adjustment than to left-right without a compensation circuit).