I used to have one, a wavy blade with a plunger on top. You put it over your onion or whatever and wail on the plunger for awhile. You end up with vegi bits that are rather uneven in size, and a bit crushed because the blades aren't really sharp enough. Then you get to try to clean the goddamned thing and try to find someplace to store the clumsy klutz.
To chop vegetables you take a razor sharp knife and slice them thin in one direction, then slice the slices in another direction - and a third if its intact enough. Then you do a little chopping. Onions are particularly easy because they're pre-sliced in one direction.
Knives are very easy to clean and store neatly on a rack.