This specific antifungal does not have much documented research in relation to seborrheic dermatitis. Thus, it is not often prescribed by medical professionals.
However, a reader of the website indicated that a dermatologist in his home country of Denmark prescribed this for treatment. His results were superior to anything else he has attempted, and several readers have confirmed similar results.
Nystatin is a fairly popular anti-fungal agent in medicine and is included in the World Health Organization list of essential medicines (source). Based on the examined research, it appears to be safer than other commonly prescribed antifungal agents because of its minimal absorption through mucocutaneous (where mucosa transitions to skin) membranes (source).
One thing to note though, is that there appear to be nystatin resistant strains of malassezia (source). So this could explain why it is not one of the most commonly prescribed treatments for seborrheic dermatitis.