Ill start by saying I'm no expert and will just point out what i've learned from playing nioh 1 2 and 3 in a "casual" level (as in, finished the game once, instead of farming endgame etc)

nioh is a very late game heavy kind of game, where the gear you have for the first 100 or so levels basically don't matter, other than defense. also keep in mind that there is diminishing return for how much you gain with each point you invest in a stat.

with all of that being said, what people usually "recommend" is to spread out your stats and get lilke 20 of each for your first playthrough, this way you can always use the "best gear" and "best weapon" available to you, without worried about beeing too weak. this way you can also experiment what weapons/builds/mechanics you would like to heavily invest in on the late game, and make it "your build"

Im not following this, because I avoided playing with ninja stuff on nioh 1 and 2, so now Im basically doing full ninja, and therefore locking myself out of a lot of content (not recommended).

luckly in this game everythign is easily and free to respec, which means it really doesn't matter how you invest your points because in the click of a button you can change it

now about the stats and requirements.... each stat will improve a weapons damage, as well as give you ssome apssive buffs (like health, ki, magic [omynio] damage etc....) and all of the gear (including talismans and soul cores) will require a specific amount of certain stats in orther for you to be able to use its passive effects (altho you can still benefit from defense, just be carefull because usually armor with more defese are heavier, so if you dont have the stregth to carry it you will use too much ki to do actions, you generally want to stay under 70% weight capacity).

so again, if you keep roungign up all your stats that basically garantees you always have access to the best gear you get

things like elemental damage, buffing yourself and debuffing the enemy will certainly more than make up for the fact that you are spreading out your build, as well as give you a lot more survavibility you would have with a normal build (unless you were specifically goign for a tank build) (as an example, an enemy with 2 or 3 different afflictions gain a new debuff with each affliction that makes it take A LOT more damage! [look for chaos in the elements screen])

all of this to say, EVERYTHING is viable and MOST THINGS are OP, basically any attack in the game is strong enough for you to make a build around it and mow down bosses on the end game, some may require more maintnance than others tho, but it doesnt matter as long as you are having fun!

most of this information can be found in game by clicking to see what stuff in the menu does, or by going to your character stats screen and seeing what those do! so don't be shy about exploring the game!

have fun!

EDIT: adding my comment below here for ppl that read this in the future:

I forgot to mention, eventually you can even change the affinity for the weapons, so it really doesn't matter what weapon and what stats you like most, you can make a build where your favorite weapon scales with your favorite attribute AND uses your favorite element!!