No, it stays voluntary. You can choose to use it or not to. But if you do choose to use it, there is the chance of failure.
And the difficulty does scale... in a way. By doing less damage.
In other words, showing restraint.
Using the saber one-handed and refraining from using damage-boosting powers. The cost being the effectiveness in the attack itself. Or making sure that your Use the Force roll can compensate.
Obviously, sparing someone with a whirly light blade of instant bisecting shouldn't
be easy. Nor should it always be more practical.
Although it might make more sense for the DC to be altered by attack rolls...
Also, kckolbe; that's already how it is. Must excede both current HP and threshold.