There's a great deal of confusion between offset disc's, disc tillers, and disc plows.
Offset disc's are a Vee-shaped disc, and not the double chevron shape of a standard disc harrow. Generally, they're very heavy, with wider spacing between blades, and made for primary tillage, generally around stumps, roots' rocks, or rough ground where a conventional plow would struggle. Sometimes an offset disc is referred to as a bog disc or a bush & bog disc. They were intended for clearing ground where heavy roots existed, or to aerify muddy ground.
Disc tillers were a rarity in most areas. They were primarily used in wheat country. They had large disc blades, but were arrainged in a single row, usually angling back away from the tractor. They were used to turn under crop stubble and residue. They required less HP per foot than a moldboard plow of the same width. Sometimes they were used AFTER a moldboard plow as a first pass before finishing with a conventional disc. They were good at breaking up turned sod when used in that fashion. A disc tiller usually didn't work the soil nearly as deep as a disc plow.
A disc plow is a primary tillage tool. They took a LITTLE less hp per foot than a moldboard plow, but not significantly less. They were good at dealing with sod, heavy soils, and burying heavy crop residue. They worked better in some soils than others. The disc that did the plowing was usually quite a bit larger diameter than the blades on a disc tiller.