I agree that 12 h on and 12 h off may be difficult with some horses who hate having the muzzle on. I have had 2 of those, and they spent 12 h in a dry lot and 12 h out, but both, if they saw me coming with the muzzle would try to do avoidance techniques (sometimes for 5-10 minutes). With those two, I learned to feed them their grain, hide my muzzle in a big bucket (that looked like it was a feed bucket) and wait till they finished their grain and immediately put the muzzle on them. The also did not give me sad eyes but rather I hate you eyes!
I have a another mare who could care less about whether I put the muzzle on and she is always easy to catch. Sometimes she wears it 24/7, other times 12/12, sometimes not at all depending on her weight.
One of the mares noted above (who hated it) was hard to train to the muzzle. She refused to eat with it for days and days. I would take her out and shove food/grass through the hole and she would eat, but the moment I left, she just stood and sulked. I left it on for 4 h on d1, 5 h on d2, 6 h on d3, etc, and finally, after about 4-5 days she actually went out and tried to graze. In the end she was fine (although she hated the muzzle to be put on).
So as everyone said, each horse is different. But it is much preferable to have them in the muzzle vs getting too fat.
I can imagine the equine end of this conversation: "all humans are different and some of them are easier to train than others, but if you sulk, make faces and pitch a fit for long enough, you CAN train them to accept the no-muzzling law".
I drop a couple of horse cookies in the muzzle as it goes on. No fits around my barn about the muzzles. they are happy to let me put them on now...no more chasing the old guy around the stall as he rolls his eyes in "terror."