Been playing this the last few days. Haven't gotten deep enough to evaluate level design, story, etc, mostly getting used to the combat system. I am playing with mouse and keyboard, and it took me like a full day to remap all the important keys. The default Kb&M controls are trash. This game also has more controls than any other non-Dwarf Fortress game I've ever played.
The combat is brutal. It has some elements of Dark Souls combat, but overall, it's much faster and more difficult. Dark Souls combat is more tactical, in the sense that you learn different enemies' patterns, and can play against that. In Nioh, there is some of that, but in addition to it, your reflexes are also tested. In Dark Souls 1, the only enemies that really tested your reflexes were skellies, those thieves, and Black Knights, but in Nioh, you run into a ton of such enemies (Revenants, Samurai with Swords and Spears early on). They attack super fast, with different patterns and feints, and attack constantly. Since my reflexes are kinda shit now that im no longer a spring chicken, it took me many tries to defeat most of these enemies. Eventually, you do get better at it (which is a big part of the appeal of these types of games), but still, it's pretty brutal.
My current build is a regular katana samurai, mid stance for counter gameplay, and using Haze to parry. Will eventually get Haze 2 for the follow up knock-down thrust, and combine it with Iai Quickdraw 2 for a vicious combo (parry, thrust to knock down, charge up quick draw and massive final blow). I also switch to low stance for multiple opponents (dodge is crisper and cooler looking), and use bow when needed.