You can if you've taken responsibility for the life of an animal IMHO. I had a dog that had a severe allergic reaction once and he would have died if I hadn't had a vehicle to get him to the vet. For myself, I wouldn't hesitate to live in the middle of nowhere by myself. But it's different when you have a pet. If my horse colicked because I was late coming home from work and there was no one else to feed him he might end up OK but I would feel guilty anyway.
I keep my dog inside when I'm not home, so I don't have to worry about her getting into anything that could give her an allergic reaction when I'm not there. So that point is moot.
Don't let your dog run loose, because that's as irresponsible as an owner can get. I'll bet there are leash and license laws in your area that make running loose illegal.
Your horses can colic at any time, whether or not you're there. They can also colic and the BO/BM not be there, or in the middle of the night when the caregivers are asleep. So that point is also moot.
You just can't plan for every contingency and there are going to be times, even if your horses are boarded, when there are NOT going to be any humans around.
Make sure your horses have a consistent diet and consistent feeding times, abundant clean water available, and keep them in safe fencing. That's all your BO/BM is doing, and anyone dedicated enough can do the same.
I refuse to live in fear and guilt, because that way lies madness.