Difficult in what terms? How would you describe the reason for the difficulty?
The range of locations both of nage and staff at any given moment during the shomen is so tiny that in practical terms a range doesn't exist, only one location both in time and space is possible. Any deviation or error will cause the shomen to fail.
Medium or high frequency activations are also impossible when done correctly due to the nature of nage's structure. The strike's energy 'wants' to disperse, only a certain level of perfection will allow it to concentrate for lower frequency activations. The psychological implications are immense and it's by far the most humbling form of all.