This thread makes me think about a video a colleague took years ago on a research project. The field was being
protected by propane canons.
In reviewing the footage, it was possible for them to see that right before the firing of the timed blast, there was
an audible sound the deer could hear.
They would simply brace themselves for a moment and once it blasted, they'd go back to grazing.....
Any deterrent is only as good as it can be when facing not only species who learn quickly and adapt and adjust, but also
what is the pressure level in terms of how many similar food items are available in the area? Is every house on the street lined with similar tasty foliage, or are you the only ones? If you are the only ones or one of few, then that pressure may be high enough that even being physically assailed isn't going to stop them from raiding the yard.
For some animals, some types of deterrents are offensive enough that they can have enough impact to scatter them or keep the pressure off.
I like the water scarecrows for animals who don't want to be wet or hit with water (skunks) but for deer, raccoon, etc... I'd keep my cash in my wallet!
The most foolproof long term option is to adjust what you plant to unpalatable items, native plants who have evolved with the local wildlife and have developed nasty bitter tastes they don't care for, etc...
Or, adopt a rescue dog, install a good invisible fence and let the pup do the work! There are a fair amount organic and other farms who have trained dogs to a perimeter and allow them to do the work of hazing and harassing the wildlife.
Lots of cash going into deterrents, every day, all over the country... Somewhere there is a guy on a private island built on plastic owls, mothballs, and of course sonic deterrents!