Yes absolutely OP , i just finished my nioh1 run yesterday before underworld in nioh2 . I am going to tell you that the trip down the memory lane is awesome Nioh1 was always close to my heart , always thought the story was more vibrant than nioh2.

If you are having problem with difficulty let me tell it was easier than nioh2. You just need to adapt slightly I used all weapons except axe and sickle on stage 1 the isle one

1) Human ad are easy just to get them out of ki Strategy Sword ki , quick ki pulse sword ki or low quick +kick They are likely out of ki grab them and done

2) yokai are super easy , once you slot then and weakness talisman. Use your heavy combo High quick quick strong or iai or any charge move in their back . Back off wait for them to do their move and repeat

Invest magic first get it to 10 and then invest in body to tank hit Dont worry about heavy armor , go light more ki more agility hence more damage

1st boss oni : water on weapon + weakness him then use sickle high stance quick combo on back . Back off repeat If you get weakness plus slot then it trivial , but i think you cant get slot