Been used on tractor pto's since, well, since the PTO was invented. Not many deaths and arms removed.Looks lethal....
Biggest hazard is getting loose clothing caught and then getting wound up. No e-stop on a tractor PTO; takes more than arm to stop a diesel engine.
This guys setup is actually safer - it has a rudimentary e-stop.
Plenty of really dangerous home made contraptions on you tube.
Here's a starter.
Just search for "homemade wood cutting machines"