How is it more difficult? The 'Robben' scenario is actually where the attacker has the upper hand and is favorably driving at the fullback from an angle with the defender on the back foot. With superior acceleration and proper execution, it's just a matter of cutting into the space on the inside of the defender or feinting back to the outside. Both maneuvers are in the same general direction (with respect to 180 degrees) so you go with your momentum and are in constant sight/control of your man.
Iniesta's dribbling scenarios often require twisting and turning to retain the ball with the pressure of 2/3 guys around him. It takes a huge amount of close control to not turn the ball over and a high level of balance to keep dribbling while there are bodies pushing at your back and feet jabbing at you from every direction. You have to have the awareness to ward off the pressure of your markers even when you can't directly see them and you have to create the space that you're dribbling into. What makes it more difficult is that you have to be moving in several different directions (forward, back, turning, feinting left/right) rather than just sprinting in one direction and then pushing the ball to the left/right. It takes more skill, balance, and control to perform Iniesta's dribbling maneuvers though acceleration isn't as imperative even if important.
Besides, Hazard is often found doing both. There are plenty of examples of him harassing fullbacks by cutting in/out of them as he runs from an angle on the wing AND instances of him performing neat turns and touches to dribble out of tight scenarios in the middle surrounded by bodies.