They are not immediate.
The delayed effect, for example, the long incubation period for some agents, may detract and limit their tactful usefulness as a political statement.
They are hazardous to all who come in contact.
There is the possibility that the biological agents could also affect the health of the aggressor forces.
They are hard to control.
The dependence of prevailing winds and other weather conditions such as temperature, sunlight, and desiccation may make it difficult to control distribution of the biological agent.
Potential long term effects beyond the initial attack.
The persistence of some agents such as spore-forming anthrax in the environment may make an area uninhabitable to aggressor forces for long periods.
Results are unpredictable.
Morbidity secondary to a biological attack is unpredictable since casualties will be related to the quantity and manner of exposure plus the preventive and treatment measures available.