That's assuming they keep up on any necessary calibration and maintenance, as well as proper training. Just seems like another potential avenue for abuse and/or false positives.
Calibration and maintenance of electronic devices is much more transparent. Defense attorneys successfully challenge breathalyzers and similar instruments all the time.
One reason dogs are unlikely to be replaced by a device is precisely because 1) dogs are opaque instruments not readily susceptible to critical analysis, and 2) have long been given the benefit of a doubt--much like cops' famed mind-reading faculties. From the perspective of the police, it's a don't break what isn't broken situation.
