Let's face it: the term "juice cleanse" isn't exactly a scientific one. It's really only a "cleanse" if your intention is to rest your body and encourage it to release whatever toxins it can. Even if your mind is at ease and you're reducing stress, there's a kind of cleansing effect happening. But if you're cutting calories to shed pounds, well, let's just call it what it is—a Juice Fast.
From my personal experience with doing long juice fasts and facilitating tens of thousands of juice cleanses during my time at my juice company—which, at its peak, had 85 stores during the juice cleanse/fast craze—I can tell you that the worst failure in the industry was simply that some folks just felt hungry and couldn't mentally adjust to not chewing food or eating out of habit. Out of the thousands of testimonials I received over 14 years in the Juice Cleanse Fast business, 99% of people I met loved juicing.
The biggest hurdle for the mainstream public? Juice costs around $60 a day, which many people find too expensive nowadays. No judgment here—just my observations.