What is wrong with the Roman Vallus?

The famous Roman or "Gallic" Vallus, an early reaper/mower style tool for harvesting small grains. It is only represented in a few text sources (Pliny Natural History and Palladius De Rustica )and a handful of carvings (like the one above). The reconstruction above looks like a very ineffectual tool for harvesting anything but frustration. My guess is that a tool important enough to be talked about by major authors and carved into stone, probably worked reasonably well in certain conditions. Like many ancient inventions, a little inference from common sense might produce a more accurate (and workable)model of this famous machine.