It's all about how high the pyrethrin count is in the fly spray. Pyrethrins are the organic compounds that have the insecticide ability. Ultrashield EX has some of the highest concentration of pyrethrins, around .50%, so yes it's at the top of the most efficient fly sprays. Bronco, on the other hand, has only about .06% pyrethrins, and doesn't do much to repel flies (which is why it's cheap). Check the front of any flyspray bottle, it will show what the pyrethrin percentage is. If you want to use an all natural spray, I've had good results with Eqyss Marigold, but it needs to be be re-applied every two to three days. I use Ultrashield EX and Equispot, which is applied once every two weeks. My horse is always fly free and happy that way.
The owners of the tack store where I buy most of my horse supplies at , suggested using Endure, it worked really well for them and they have 5 horses. I tried it last summer on my geldings and it worked well for me. I try to get it when I see it on sale or have a coupon sometimes at Tractor Supply it will be on sale, it is a little pricey but it worked for my horses