How do they handle roots, sticks, etc...? What happens when you hit an old stump or rock: shear a pin? bend a tine?
I've yet to hurt mine. It's not that tilling is slower than discing, you just dont have a high travel speed. One pass with tiller is all you really need. A 100 hp tractor with a 3 ton disc is nice but most people dont have that to work with. You put a tiller behind a 20 hp tractor and your in business.
