There is very little warning when it falls, maybe one small sound, and then a huge thud. I can see the ice start to advance over the edge when there is a warming spell. I do not ever walk in or under the eave area where the ice is probably going to fall. And I have signs warning others to not do so either.
I also do not let the dog go out in the yard when there is a warm spell and ice is still on the roof.
Yes, it can kill someone.
The snow slides work very well, but the above is the trade off. We had several years when there was flooding inside the house because of ice dams backing up water on the roof, and one of the floods tore down an interior wall. So this has stopped it.
A lot of people here have electrical cables at the roof edge, but they don't work very well, others climb up on the roof and manually chip the ice off--dangerous. Or they use special long roof rakes.