I have one concern with the heating pads in the hen house.
I'm pretty careful to only use electric items that are made for outdoors and wet situations. I know the heat pad wiring wouldn't be rated for that kind of use so I get a bit concerned about using them with the dust/wood chips, etc. that is in the chicken house.
It's not the temperature I'm concerned with but shorting in the electric wiring itself.
Now...if I were brooding in the house I wouldn't be quite as concerned.
Actually, most heating pads are built for using around moist compresses for therapy reasons. The one I'm using is completely machine washable. They are quite possibly way more safe than heating designed for livestock for the simple reason they are designed to be used on humans, which causes them to earn a much greater safety rating even when not used for just house use~humans do crazy things with home medical equipment. I'd be far less concerned about a heating pad shorting out than I would a heating lamp, heated dog bowl or bucket, etc.
The Sunbeam pad I'm using lists it as for use with moist or dry heat, so it's fully within the safety functioning of the pad to use it under moist conditions. I think any person using any electrical devise should use common sense with operating it, so that is up to the individual how they position their pads for safety. Home brooders use the same components...water, shavings, dust, feed chicks, etc. I think if I had concerns at all about an electrical devise under those conditions I'd rather it be in the hen house than my own home if fire was the fear.