There are certain disadvantages:
A. Collective behavior cannot be inferred from individual agent behaviour. This
implies that observing single agents will not necessarily allow swarm-defeating
behavior to be chosen. (This can be viewed as an advantage too from an
aggressive point of view).
B. Individual behavior looks like noise as action choice is stochastic.
C. Designing swarm-based systems is hard. There are almost no analytical
mechanisms for design.
D. Parameters can have a dramatic effect on the emergence (or not) of
collective behavior.