It's not that the enemy variety is low, it's just the game is very big.
It's actually not that big if you don't go for side missions or NG+. I've seen people go through the entire game in 30-50 hours, with the DLC missions included, which is similar to the length of an average Souls game. The only thing you miss out on by ignoring side missions is the occasional "human samurai but with some unique magic" boss fight, or, in the case of the DLC areas, a couple of cool optional bosses like Ninja Gaiden man and the revamped ninja frog fight from Nioh 1.
DMC and Bayonetta are beat 'em ups, not action RPGs like Nioh. Not comparable. Raw numbers don't tell the whole story either, it doesn't matter if a game has 300 unique enemies if you fight a small number of those for the majority of the time.
Boss variety is a separate issue entirely, but Nioh and Nioh 2 are fine in this aspect. The bosses were the only thing keeping me playing Nioh 2 as long as I did until I finally got sick of the boring levels.
Nioh has a pretty similar gameplay structure to games like DMC and Bayonetta so I don't think you should dismiss the comparison just because in Nioh your numbers go up.
It's a mission based game, where you can replay any mission any number of times, not unlike DMC/Bayo. After you beat the game you unlock a new difficulty that not only makes the enemies stronger, but also changes enemy placements and gives them new moves, again, not unlike DMC. As you play through the missions, the game introduces new enemies, and then reuses those enemies in subsequent missions, similar to those games as well.
For example, you play DMC 5, in one of the missions you go through the level fighting mostly the bug enemies and the scythe and cleaver demons. Then you enter an abandoned library and the game introduces the ghost with scissors thing that flies through walls and you fight it in the library, and appropriate place for a mob like this. But afterwards the game will just keep reusing those ghosts in regular combat arenas together with the other mobs. Nioh games work exactly the same.
I also think it's not fair to say that you only fight a small part of those mobs on a regular basis. Other than the miniboss-tier enemies like the giant bull demons and dark magic birds, pretty much every single demon that has already been introduced gets encountered on a regular basis in almost every mission at least once or twice.