According to FDA 21 CFR Part 184 (Direct Food Substances Affirmed as Generally Recognized as Safe), DAP is considered GRAS by the FDA for miscellaneous and general-purpose direct usage in food products, with no limitation other than that imposed by Good Manufacturing Practices.

Is diammonium phosphate harmful?

The toxic effect of fertilizer Diammonium phosphate resulted in alterations of 5'-Nucleotidase activity of tissues--liver, kidney and muscles of fish C. batrachus at varying intervals and exposures. Alterations in 5'-Nucleotidase activity of body organs gave an idea of the toxicity caused by the fertilizer.

Why is DAP bad?

DAP can increase the risk of volatile acidity, and some believe too hearty of a fermentation can mute aromatics and flavors. There's also a risk that the fermentation will be too robust, creating microbial instability and paving the path for spoilage organisms.