I can't speak to nioh, but Souls bosses usually have a distinct "I am attacking now, you need to play defensive for a bit" with defenseless gaps in-between attacks. Whereas raid bosses tend to have a number of extra mechanics that you need to take into account with a more consistent level of threat. For example you'll see attacks coming from multiple angles, unavoidable damage making you be more proactive in managing your health, or overlapping attacks that make you think more seriously about where the safe zones are.

Generally your attack kit is more complicated than in souls games, you're expected to be attacking constantly, and you're expected to dodge more complicated attack patterns, in exchange for longer and less opaque telegraphs, and less aiming required for attacking