There are very isolated instances where folks have really clean power and do not benefit from it.
The Niagara power conditioners stop noise, that is generated from each stereo component, from traveling to the other stereo components, which results in a lower noise floor.
So even with very clean utility power, any reasonably good stereo should benefit from the resulting lower noise floor.
I wonder why some stereos do not benefit from the Niagara?
Perhaps they are using a mass produced power cord?
It is my understanding that the most important power cord, is the one which connects the Niagara to the wall. That power cord (the right one) is expensive, and there might be some people that do not believe in those power cords being beneficial?
It is also my understanding that for the Niagara boxes, not just any power cord will suffice. I believe that Audioquest has specific power cords for that critical purpose of powering the Niagara box.
I believe that the correct power cord has special grounding properties, and also contains materials that store energy, which helps when there is a transient demand for instant power.
Any responsible dealer would have told their customer. It is possible that someone sold their Niagara, and the new owner was not aware that it is important to have the purpose-built power cord, too.
The above are things that I have read about with Shunyata Research's Everest power distributor, and I am assuming that it likely applies to Audioquest, too.

