The thresholds can be set according to jurisdiction, and I do have confidence in the machine. Again, the way I see this being used, the most effective use of it, would be for a police officer to use their own senses but then also have the machine. The feature is very simple. You push a button, it sucks in a bit of air, you hear the fan, and it gives you the reading.
Between those two, if alcohol is there, I believe you have a high probability of establishing those grounds.