Be sure not to add the ice cubes until the very end. If the dates are too cold when they are blended they will clump and not blend well.
You can substitute other nuts for pistachios. We’ve had success with soaked almonds (raw, not roasted), cashews and pecans.
Medjool dates are by far the best for this smoothie. If you use a different kind you might try soaking them in the milk for a few hours ahead of times, or even cooking them into a syrup first, to make sure they blend well.