While the game visual design is a bit lacking, compared to souls series and enemy variety is definetly one of the lower points of the game. They seem such petty reasons to stop playing a game, specially if you enjoy the combat. (The narrative sure is lacking, but the game has like 10 minutes of cutscenes in like 70h of gameplay, is such a non-issue...)
Level design is fine, most souls game level design is flat too (I guess you mean no verticality?) and later levels in Nioh offers some cool layouts.
One thing is to expect a souls-like experience and another thing expect an exact copy of the series. If you want more souls game, there's a DLC incoming. Meanwhile I recommend you to be ready to not play another souls game in years, and not finding exact copy-paste games to fill the gap.
I actually admired the way they've managed to diversify the game from the Souls series - particuarly in terms of combat - and I do credit the game for this. In many respects it doesn't feel or play like a Souls-clone and that's great.
However, the level design is a fairly direct attempt at a Souls knock-off, right up to kick-down ladders, and doors that 'don't open from this side'. The risk reward sense of gameplay that Souls captures, by giving you huge labyrinthine spaces and daring you to explore them, is perfectly balanced to create that constant sense of dread - right from the very outset.
Nioh doesn't capture this. In the first instance the level enemies are actually a bit too easy to tackle, meaning the sense of trepidation rarely grips you and the lack of enemy variety means you just don't turn a corner and find yourself confronted with something you weren't expecting: think of the Crystal Lizard from the opening level of Dark Souls 3 or the Executioner on Yharnam's little backstreet.
Moreover, they tend to be quite open, allowing you to view whole areas at a distance, headshotting enemies with ranged weapons or sneaking around. Souls design constantly blocks line of sight to prevent this kind of reconnaisance and planning: think of Yharman's main drag between the clinic and the giant pyre. On it's own, it's a straight road you should be able to see most of the way along, but the designers deliberately blocked line-of-sight with abandoned carriages and debris and it keeps that sense of claustrophia and dread dialled up.
Nioh's fishing villiage however offers a much wider space, where guard patrols can easily be observed from a distance and enemies picked off with bows and arrows. It's rare to turn any corners and even rarer to find something unexpected around one. So yeah, that covers my thoughts on level design - I think it's the area where Nioh most closely apes the Souls series and were it misses the mark most noticeably.
That said, I'm not looking for another Souls game, I'm just looking for good, rewarding gaming experiences and they can come in all shapes and sizes.
