I made one of these branch choppers some time ago. It's great to repurpose the branches for wood stoves, wood boilers ,etc that otherwise would simply get burned or piled somewhere, taking years to decompose.
It's getting harder and harder to do any pile burns, so this type of attachments are great to get rid of piles of branches.
For those so concerned about safety, it's no more dangerous than a wood
chipper really. On a wood
chipper you don't stick your arm inside the funnel either, do you? Just simple common sense. Yes, one the videos it may look sketchy because people are still testing the thing out before the finish the build with all the proper safety covers.
The material that comes out of it, is great for the BBQ too. Now, knowing what I know now, I would probably have built a wood
chipper, mostly because the chips will decompose faster and I don't burn much wood anymore, since we went solar water heating some time ago and recently we bought a pellet stove.
Some pictures. This thing has eaten stuff up to 3" of various types of wood.