Not terribly much to say about Yin State by itself. The heightened spiritual defences could justify picking the Yin specialty on their own to be honest. The immunity to regular to A-Rank and below Genjutsu, and S-Rank or lower spiritual techniques (that aren't based on Yin Release or a Dojutsu, like the Yamanaka techniques) is incredibly useful.
Before I give my thoughts on Kuninotokotachi, I wanna clarify that there's two features to this technique; the ability to prevent Genjutsu break via subtle mental suggestion that forces a target not to deduce the illusion, and a powerful, albeit delayed, hypnotic effect that KO's a target provided they're within range. That's how this reads to me. That being said, while its effects are potent, it encourages you to stay within short range to hasten the hypnotism. Against people who have an arsenal built around CQC, and with Yin State restricting access to Taijutsu and other fields which can increase your effectiveness in that short range, it's potentially quite risky.
Before I give my thoughts on Kuninotokotachi, I wanna clarify that there's two features to this technique; the ability to prevent Genjutsu break via subtle mental suggestion that forces a target not to deduce the illusion, and a powerful, albeit delayed, hypnotic effect that KO's a target provided they're within range. That's how this reads to me. That being said, while its effects are potent, it encourages you to stay within short range to hasten the hypnotism. Against people who have an arsenal built around CQC, and with Yin State restricting access to Taijutsu and other fields which can increase your effectiveness in that short range, it's potentially quite risky.