Aside from the ice/drowning issues that you have in your region, the problem I have seen with ponds that are standing, they become stagnant. The deer stand in them to drink, and they pee and poop while they are drinking. Before long, unless it is a really big pond, more of a lake, or unless you have a spring or creek that is constantly moving fresh water through, is that you end up with more mouth infections and e coli problems because of the extra bacteria that gets ingested. Same will happen with pens that don't drain well after a heavy rain.