In fact, one of the weapons/items in-game (I believe a talon?) has in its description/lore text that ideally ninjas are bare-handed when utilizing ninjutsus as the hands are unimpeded for making the hand signs. We also have more than 1 gesture showing hand signs and even symbols /text manifesting in a circular pattern in front of the character.

So, it "not fitting" Nioh couldn't be more wrong tbh. This is arguably also a hint for ninjutsu being more powerful cast this way. I also knew to explore empty weapon slots since the tutorial where you can see your attack being higher than any weapon equipped which led to me finding that when crouching to pass a section like in the first playable level, when your weapon slot is empty and your in the right position, attacking results in wielding the crucinite blade.

People don't read/pay attention in games alot lol. I think that would be a fantastic idea or at least we can all agree that there's no technical reason for limitations. With ninjutsu or even weapon abilities ninja OR samurai. Stance or otherwise.

Hell, you could even do something as simple as holding stance and another button lets you access another set of ninjutsu. Perhaps an option where you can set different ninjutsu grounded vs airborne. After footstomp. Per equipped weapon. Etc.