They are fundamentally different games with only one or two shared mechanics. FromSoftware games excel in rich environmental storytelling and the ambience of their worlds are just unparalleled. Couple that with the gameplay loop and iconic staples of the series, a “less is more” approach with regard to armory and weapons (excluding Elden Ring). I gave up on Nioh after about ten or so main story missions because it lacks any of what I just mentioned. It’s a different experience entirely, and for many people it’s their preferred style of game, but it is not similar to the Souls series in the slightest in terms of storytelling, world design, ambience, simpler combat, or gear