They're very safe. Really, nobody would use them if they were chopping chickens in half.
I didn't want to mess with building one from parts, as easy and cheap as everybody says they are. I have purchased three of them, two of my favorite kind and another kind when the company was out of stock when I wanted to order a third.
I prefer the battery-operated doors with solar sensor for opening and closing triggers. The batteries - 4 AA batteries - have lasted in each controller for more than a year before they needed to be changed.
The other kind I bought requires electrical power, and since I now have a coop WITH power, I ordered it. It works fine, but yes, when the power goes out, chickens can be stuck in or out of the coop, depending on when the power loss occurred. As soon as the power comes on again, the door will either open or close, depending on what state it was in when the power went out.
Battery operated doors are obviously not dependent upon electrical power, and the light sensor is wonderful at the factory setting. Just perfect.
Both models have metal tracks for metal doors, so the weather doesn't warp anything. Also, the controllers are inside the coop. If you get one with a timer to control the door, you will have to keep adjusting it for the change in the number of hours of light (seasonally). I have found the timers don't do well in dusty environments. Hello? A chicken coop? The battery operated one comes with a cover for "the works" so that doesn't happen. Also, I've had chickens jump on the timer and move the little tabs, changing the opening and closing times.