The downside to using water is that props must be thoroughly dried before they can be refueled and used again. The residue from the extinguishers makes it very difficult to re-light or refuel props so the props must be thoroughly cleaned before each use. Dry chemical extinguishers will start to lose gas after one single discharge and must be immediately serviced and refilled which is expensive. Beyond that, the powder produced when discharging a fire extinguisher is actually a bit of a hazard. The discharge must be cleaned up immediately after a discharge.
Some people find the powder to be an irritant.
Discharges a white or bluish cloud. Leaves residue that must be thoroughly cleaned after extinguishment. Dry chemical will lose gas after a single discharge. Discharges a yellow cloud. Leaves residue that must be thoroughly and quickly cleaned after extinguishment.