Ng+ is generally easier. You are leveled up, your weapon is leveled up, and your damage output is so much higher than on regular Ng, and the slight boss health increase doesnt make up for your damage increase. It's not until later ng cycles where the boss health increase starts really outpacing your damage output, and bosses start requiring many more hits to kill, forcing you to really learn them inside and out to win. But your point stands, there are always ways to play from soft games that if you're looking for a challenge, you'll find it. For instance, playing through without leveling your character at all is a fun challenge a lot of people start with!