I dont worry about such things. Humans can go several days without fluids, so can chickens. Obviously, if they are allowed to drink when they want it is better for them. I check my feed and water every day so worst case is part of a day. If i start having troible with draining i can always throw in an old drinker, eh?
DH & I are worriers - if we're gone for the whole day into night we rest easier knowing there are multiple feeders and waterers until we get home. Not so much a worry in the cool months but our SoCal summers are brutally hot/humid plus we'll throw ice cubes in the insulated jugs and the water stays cool for a few days. The nipple valve waterers are 100% better than the open container waterers we had before. The wild birds pooped in them and the chickens would stand in them like a spa -- they all thought they were "Palmolive Madge."