Dumbbell kickbacks kinda suck – there's tension only in the top contracted position. Using cables or bands to create tension throughout the entire range of motion is more effective, but not as effective as kettlebells.
Because of their unique leverage and loading mechanics, kettlebells place a huge amount of strain on the triceps, particularly on the lateral and medial heads. Although they won't give you as much continuous tension as the cable variations, the amount of activation in the contracted position is so great that it more than makes up for the momentary relaxation in the stretched position.
The key is to use a somewhat compact range of motion. Rather than extending too far, which causes the rear delts and lats to take over, try pausing in the contracted position where the arms are approximately in line or slightly below the plane of the torso.
Multiple sets of higher reps (12-20) are ideal for this unique triceps blaster.