They have relatively low false alarm rates, and a high detection capability. However, they have many difficulties, as well, that come naturally when working with a living animal. Basically, they are trying to create a mechanical reproduction of a dog's nose. To make an analog of the dog's nose, we must find out exactly how a dog is able to detect the explosive material. We do not yet know whether the dog is sensing vapor phase chemicals or is breathing in some particulates and doing a thermal desorption within the olfactory region.
Much research is underway to determine the answers.