You're spreading (often) human waste over a field. There are a lot more pathogen risks this way.
Only if it isn't treated carefully. Spray it on in fall after harvest and you are probably fine. however spray right before harvest and pathogens will still be alive. a month before harvest is often when plants need the most nutrients, and that isn't enough time for any pathogens to die.
Some pathogens simply do not die in any useful timeframe, e.g. anthrax or tetanus. Those lurk in the soil for years, decades and even centuries.
The only safe measure is heat-treatment, but that is usually too expensive. In particular, it is more expensive than just using artificial fertilizer.