Absolutely no faith. We have lots of deer here, and almost no one uses this type of device.
I read some of the Amazon reviews... yeah, deer stop and look at me sometimes, too, but I don't have the whistles. In several thousand trips through a rural area which can have deer traffic at any time of the day, any day of the year, I have hit only two; both times my fault for not paying enough attention or responding quickly enough. Some people hit deer every year. I'm not giving credit to whistles.
If they work at all, they will need a location with suitable airflow... which might not be where you expect it on the vehicle.