My horses have lived in fly masks for the last 20-some years --- just as soon as the sun and the flies come out.
Fly mask do offer some UV protection and I have a couple horses whose eyes are now light sensitive thanks to metabolic issues. When the eyes run, the flies come, and the vicious circle begins.
Knock-on-wood, I have never had anyone get hurt in them, hung up on anything, or had been stung under them, and my horses roam on large pasture with trees.
They wear them in the pouring rain and wind because I don't want something blown into their eyes.
They never come in until after dark and nobody's ever run into a fence post BUT that doesn't mean a horse with poor vision couldn't.
I wash those fly masks every single night in the washer on slow speed, cold water, a TBS of laundry soap, for 3 minutes.
The fly masks that hold up the best for me are Farnum. My only complaint with Farnum is I am not at all fond of their shimmering gold. I have four horses so I want four colors instead of having to put initials on each mask and waste my time looking for that initial.
One of my seniors has to wear a fly mask to bed every night. Sometimes his arthritis causes him to struggle to get up and he doesn't close his eyes. Twice I had to wash shavings out of his eyes; after the second time, on went the night time beauty mask. That was over four years ago and his eyes are as perfect as ever