Kuniyasu, originally from Obi in Province, was the youngest brother of Kunihiro according to the Tanaka Kakeizu, where he is recorded under the common name Saburōdayū. He traveled to Kyoto, studied under Kunihiro, and achieved great success within the school during the Keichō era and into the early period. That comparatively few works bearing his own signature are encountered is thought to be connected with the fact that his working style, the construction of his tangs, and even the character of his signature so closely resemble Kunihiro's that he is presumed to have served as a — a substitute smith producing work on Kunihiro's behalf. His signature is invariably confined to a two-character inscription, and he did not employ a court-title signature (juryō-mei); among the outstanding smiths of the lineage, Kuniyasu is the only one who did not receive such a title. Said to have been left-handed, his use of reverse file marks (-) on the tang is a major point of connoisseurship and a feature that does not appear on works signed with Kunihiro's name.
The consistently identifies Kuniyasu's forging as the most conspicuously standing and rough within the group — the so-called texture of mixed with and , with thick adhering and fine entering well. His characteristic is a mixed with and angular elements in which adheres strongly, and run conspicuously, and the exhibits a tendency — a subdued quality recognized as distinctive of this group. His workmanship reveals the domain of the superior masters, and especially the -like manner in which he is said to have most excelled: plentiful with intermittent unevenness, , and vigorous internal activities that produce a bold and rustic character. The scenic effects created by and convey an impression of antiquity. Within the school, no one other than Kuniyasu is known to have produced "Sadamune-style copies" (Sadamune ), and examples of construction are seen relatively often in his work. His rare pieces, tempered in a calm manner with as the main tone, are valued as reference material for understanding the breadth of his working range.
Kuniyasu's finest works are described as "comparable in quality to Kunihiro's masterworks," and the has termed his best the — the finest — among blades of this kind, one so excellent that "even if included among Kunihiro's works, it would appear as an outstanding example." Honma's assessment observes that Kuniyasu's forging is as bold as Kunihiro's, not inferior and in some respects even superior, and that his can be even more open and magnanimous. His blades are praised for their powerful -period , for the assertive vigor of their hardened areas, and for conveying throughout the characteristic flavor of the school's superior smiths. The recurring epithet across his designated works is one of bold conception and pronounced rustic vigor — a quality that, together with the antique flavor of his scenic effects, places Kuniyasu among the most distinctive and capable members of the lineage.