I’ved used Deep Woods OFF for years and never had any problems.
I believe it’s NOT on the shelves in tack stores for horses because IT WORKS. They make big $$$$ selling you $20/quart flyspray that lasts for 1 hour.
In the midwest where we have mosquito swarms so thick you can hear them humming, you BETTER use something that works or else your horse will get carried off into the sunset. I have never found an equine product that works against mosquitos except that Eqyss Marigold spray and it’s still not as effective as OFF.
Also for ticks - we ride in really thick woods around here. If I forget to apply OFF to myself and the horse, I will find at least 5 ticks crawling on us when I get back to the barn. If I use OFF, I don’t have any ticks. I’ve never found any other equine product that’s effective against ticks. They say they are, but they aren’t.
But in any case I use sheets, neck covers and fly masks so I can reduce the amount of chemicals needed. I spray my horses very little. Even for riding I have the Crusader Quiet Ride suit and then spray the exposed areas.