Can someone please explain the difference between Samurai and Ninja mode?
Samurai mode is traditional Nioh; 3 stances (move-sets) to switch between that trade off speed and stamina cost for damage/stamina damage to the enemy. Key feature is Ki pulsing which is a means of clearing the circular pools created by Yokai that impede stamina regen. Defence is block, dodge or stun.
Ninja mode has 1 stance but greater speed and mobility at a cost to damage dealt when the enemy is facing you. Does increases backstab damage, and has the mist ability to phase through and get to the enemy's rear, think of it as a fast reposition as you can't just do it over and over. Like Ki Pulsing you trigger it at the end of an attack or combo. Additionally this seems to clear Yokai mist pools now although it didn't in the alpha demo. Defence is block, dodge, and parry.
Both styles have their own independent armour loadouts and 2 melee and ranged weapons each.
On top of this you have Yokai skills, which when equipped give you a special ability and some sort of buff, and guardian spirit skills and the devil-trigger style transform.
The only major regression appears to be no more onmyo magic this time, which kinda makes sense as ninjutsu also offered ranged elemental magic damage and enfeebs. Its not clear whether this is gone completely or its just going to be a late-game unlock. Either way ninjutsu also offers sneak/invis tools so you can better sneak attack.
Its just the Souls stamina system built out to an enormous degree, but once you understand the basics it feels quite straightforward and manageable. Mainly because its a true ARPG where everything is stat-based and customizable down to individual affixes on weapons, armor, accessories etc. You don't need to master everything at once so long as you level up and are geared appropriately - you can stick with the subset of the options that you like.
On the other hand if you really want to go deep you can link virtually anything together and just annihilate stuff...
If you're just looking for a fast-action game with simple gearing and a skill tree this is not the series for you. There are systems on top of systems and an avalanche of gear constantly dropping Diablo-style. Standard inventory size for Nioh is 500 items, with a 6000 item storage box (!) Both of which will fill rapidly if you don't dismantle, sell, or drop on the regular.
Some people don't like this loot-heavy style, but if you are really into build-crafting its just peak. Wo Long and especially Rise Of The Ronin's do similar things but are toned-down significantly, and at least in my opinion suffered a bit as a result.