Notice the pulleys from the output from the BLACK one multiply the speed about 6:1. I bet it's difficult to get it going again if stalled. Agreed it's too light to get drive power from wheel traction. I'm a little surprised there's no heavy ballast. All the work that guy put into it looks like fun - but I bet he could cut a lot more grass setting a motor on it.
The one pulled by the ATV is probably so slow the blades need to be kept sharp but up high out of the rocks it's probably OK.
I like the projects and the challenge.
The black one was built by a member here, with description and video.
http://www.tractorbynet.com/forums/build-yourself/311436-homemade-mower.html
Bruce