A complex is a series of specific strength exercises blended into one continuous training circuit. The objective is to complete the exercises in succession, doing all the prescribed reps of one movement before proceeding to the next. Traditionally, complexes are done with barbells, but dumbbells have several advantages in this case. Try these two.
Structural asymmetries are far too common. Subtle but potentially devastating weight shifts during a squat, deadlift, or bench will create tissue overload and performance plateaus. Dumbbell complexes have the innate power to address several deficiencies in movement.
Complexes trigger a high degree of metabolic disturbance throughout the body resulting in increased fat loss. This is especially true if an athlete has a solid strength base to operate from.
Complexes can be absolutely miserable, but you can manipulate the training parameters (go lighter, limit sets and reps etc.) so that you're working more towards active recovery/regeneration qualities.
Proper rotary core development is critical for performance and long-term health. Most daily motion and exercise tends to take place in the sagittal plane with front-to-back movement. Exercises such as Pallof presses, cable chops, plank progressions, renegade rows, unilateral training/complexes can cure this pitfall. Multi-directional stability throughout the kinetic chain is essential for health and performance.
Complexes typically involve moderate to high training volume, so the rep spectrum is pretty well suited for improved motor learning.