The garlic and vinegar ideas have been around long enough to have research done that shows both of those products used on a consistant[all summer] basis over a period of time had a 170% increase in the horse developing gastric ulcers and some 1.5% liver and .075% kidney damage.
I personally use the feed through larvicides either Equitrol or MVP the research has shown these to be effective and non harmful. With kidney function being the only organ affected in .0026% of horses. Not all products are the same the Simplifly can also have side effects much higher than the others and I've found the Solitude a waste of time.
The fly predators do work, but they are to be placed on a manure stock pile.
I have all horses in my barn on it, started it March 1, most of the time there are between 8-15 horses, monthly cost $80. But have not purchased fly spray or any topical product in 3 years. I do also hang the cheap fly bags.
I can take an open coffee to feed mornings/evenings set down 5 minutes later-not a single bug.
Being a HUGE coffee drinker- thats enough of a sales pitch for me !