Difficulty means technical difficulty - how difficult it is to learn the proper form of the exercise.

Proper exercise form is vital to your success.

When you use correct exercise form, you are using correct muscles. When your form is not correct, you are using certain muscles more than they are meant to be used and other muscles less than they should be used.

If you're not performing the exercise correctly, you risk the chance of hurting yourself either immediately or in the long term. Another disadvantage is that you are not realizing the full benefits from your workouts. You are merely going through the motions. You end up using less weight, and you never progress because you just don't use the intended muscles the way they should be used.