It has been perceived that training on an unstable or "wobbly" surface is more "neurologically challenging" and therefore is best for every goal from cardio, to strength, to rehab, to any sports performance.
And some are so obsessed with it that they forget about every other way of progressing an exercise, thereby missing out on the most direct way to achieve the specific goal. They fail to realize their workouts are beginning to resemble circus acts benefiting only an extreme end of the FC (if not causing injury).
In truth, load is proportionally sacrificed as instability is increased. Understanding and appreciating the individual's goal is vital when determining which challenge should be sacrificed and which should be emphasized.
A comprehensive program would include both ends of the FC as dictated by goal and currently ability.