I've been using some of the Off skintastic on my girl's chest, as the buggies have been chewing her up there...the main thing to watch out for is hair loss, as that will indicate that it's too strong for your particular horse.
I don't like putting the chemicals on my horses in the first place, but when the bugs get bad I do. But... only with the repellent made for horses if I do have to use the chemicals I'd much rather be 100% certain there specific for my horses.
I get scared of them having reactions. I'm just over worried though. I just wont put OFF on my horses ever.
^ There are human fly repellants made with some of the same repellants as horse ones, so you could check some of those out if you ever got curious.
Apple Cider Vinegar is a good way of keeping pests at bay too...you can put some in their feed daily (which is what I do to my mare, although she's been off of it for a little while, cause hubby ran over my new gallon of it, and I haven't gotten a new one). Or you can put it in some of their drinking water, but you have to make sure to change that daily, and that can get annoying, which is why I feed it. My girl loves it!