Feathers are good insulators, scaly feet I don't think so. For something to get shocked the electric circuit has to be closed. I use electric netting where the soil is the ground and the horizontal strands are the hot wires, except the lowest horizontal wire. The bottom horizontal is not hot. My baby chicks can go through that netting without a problem until they are about 7 weeks old. They hop up off the ground and walk across the hot wires., squeezing through that hole The circuit is not closed as they are not touching the soil, which is my ground.
When my adults are standing on the soil and peck at something in the fence and their beak or wattles touch a hot wire, they squawk and jump back, then go about their business. They are not injured and learn to not peck in that area. If scaly feet were insulators they would not get shocked that way because the circuit would not have been completed. Watching this is what convinces me that scaly feet are not good insulators.
My voltage tester maxes out at 7,000 volts. I have at least that when it's hooked up and grass and weeds are not grounding out the netting. I don't even use my tester any more. When I put the netting back up after mowing the grass and weeds growing up into the netting I I test that I got the connections right by touching a hot wire. I can't tell the voltage but I can sure tell that it is hot.
On the safety of these devices. Any time you fool with electricity there is danger. That includes all the electric devices and lighting in your house that are not designed to pulse for safety. You do need to be careful with any of these devices, including these chargers. But the companies that make these chargers have lawyers on staff that look at liability issues. If they put out a device that killed a young kid by them accidentally touching it they'd go bankrupt from the lawsuits. It would make the news. Anybody that has had kids knows they are not going to avoid touching a hot fence because somebody tells them not to. They might avoid it after they touch it. If horses, goats, or other livestock were injured by touching these fences people would not use them. Think about that. If I were worried about it killing or injuring my chickens I would not use it and I don't think anyone else with livestock would either. Yet these things are standard to enclose livestock almost everywhere if you get into livestock country. And to stop predators. I do not lay awake at night worrying that one of my chickens, a neighbor's dog, or one of my grandkids will die or be injured if they touch the fence.
I have found three turtles trapped in the fence. They don't jump back when they get shocked. Two of those turtles walked away when I turned the current off and untangled them. It took them a bit to recover but they were gone when I came back. You could see them jerk every time the current pulsed but they walked way. The third died. I assume it had been in there many hours. I found a dead snake in that netting. It got tangled and could not get away between pulses. I found a possum tangled in the netting one time. Possum are really slow. It got tangled up and could not get loose. The electricity is not what killed that possum or any of the babies in her pouch. You do need to treat knives, barbed wire, gasoline, and electricity with respect, any of them can be dangerous. But don't fear them. In my opinion things are a lot less likely to get tangled in an electric fence than they are in my netting.
