This is my opinion.
Nystatin is a very valuable tool in treating systemic yeast because in these cases, you are looking at a system where yeast has breached the colon. Another term for this is 'leaky gut.' In these instances, Nystatin which reaches the colon will likely be able to reach the yeast elsewhere in the system because it will go through the leaks created by the yeast. Those areas that the Nystatin cannot reach will be affected by the improvement in the body's immune response as the yeast the Nystatin CAN reach are killed.
HOWEVER, Nystatin alone is not a sufficient treatment for yeast overgrowth. Using processed and dried foods as staples in your diet, refusing to consume fermented and/or cultured foods and/or probiotics, drinking unfiltered water, overuse of anti-biotics, and the continued consumption of foods that have been chemically treated will almost guarantee that your weakened immune system will be overwhelmed by yeast AGAIN, before your immune response has a chance to re-establish its balance.
Your body NEEDs you to cultivate beneficial bacteria and enzymes in your digestive tract (sinus, mouth, stomach, small & large intestines) or the advantageous yeast will dominate, consuming its own waste and causing ever-worsening symptoms.
Even if you attempt to starve yeast of its favorite foods, it will simply lie in wait and grow when there is food for it to consume. So, that's why I don't recommend long-term yeast or candida starving type diets. IMO, the only real hope lies in populating our bodies with beneficial flora after having cleared the area with something like Nystatin (or similar acting treatment). Keep in mind that this should be accompanied by a thorough bowel cleanse as the dead yeast cells must be removed before they ferment and/or provide food for those still living there. Once you've begun to consume good pro-biotic types of foods/supplements, the 'good guys' must then be fed on quality foods as I've described. Yes, yeast will be present, but the balance will be correct and beneficial.
The things your body needs (the good flora) cannot live in your gut on a diet of processed sugars, processed carbohydrates, packaged breads (crackers, cookies, etc.), and chemical preservatives and coloring agents -- but YEAST can and will survive and flourish in such an environment. So, it's a process: knock back the overgrowth, clean out the garbage, plant good stuff, and provide nourishment that will enable the proper balance to flourish.
Again, these are just my observations and conclusions I've drawn.
Nystatin is not a magic yeast cure pill. It is a tool to be used as part of a process to re-establish a healthy balance in the body and restore proper function.
Just my $.02