Yeah, I've seen the argument that the first playthrough is just a tutorial a couple of times, and I don't buy it at all. I tried Nioh 1's NG+ for about 3 or 4 hours and it wasn't for me, yet I felt very much fulfilled by that game.
The first playthrough in soul-like games is the best one! It's the one where you're terrified and discovering everything for the first time. You have to take your time and savour that one!
Yes, a number of Nioh's systems won't matter the first time around, but those systems don't really matter because all they effect are damage numbers, they don't have any meaningful impact on your experience. You can still enjoy all the spells, weapons and ninja abilities.
On co-op, I agree to an extent. I use it when it feels appropriate. Last night the game had what I consider to be its first mis-step, at the start of area 2. I beat the first boss, who was fantastic. The very next sub-mission ended with the very same boss. I'd have been more than happy to fight him again later on, but at that point, considering the fact I'd just spent 30 minutes clearing out the rest of the mission, I just wanted him gone, so I called some help. Really odd design decision, and hopefully it'll be a rare hiccup.