The reason it makes rhinoplasty more difficult is because of the fracture lines. When a person gets a broken nose, it does not break in an aesthetically pleasing manner. They mostly break according to the direction and magnitude of force doing the breaking. And then they heal into that broken position making the osteotomies (bone cuts) of rhinoplasty more difficult to control.
There is also the question of the airway after a nasal fracture. If the airway is good before surgery, it can be maintained with proper operative technique. However, if the airway is bad before rhinoplasty, it most likely will be the same or worse after surgery. This makes it essential to diagnoses airway problems before surgery so that they can be corrected or at least improved rather than making them worse.
Here is a video about one of my patients who broker her nose and wanted to get things back into shape:
Nasal Fracture Patient