Well I haven't watched it or read it so this is second hand, but...
It's a "isekai" series, a.k.a. a series where the protagonist is transported to another world in some fashion. Specifically it's of the "the protagonist dies in the real world and gets reincarnated in a fantasy world, swords, magic, etc. Those are a dime a dozen these days, so that's not particularly remarkable. What is remarkable is that it's older than people realize as it's one of the first popular isekai web novels, giving it an advantage after it got a proper publishing deal, got an anime deal etc. Higher expectations, so a bit more money and effort poured into it than it otherwise might have.
The catch that has some people loathing it is the part of the premise about what kind of person the protagonist is. The reasonable general idea apparently was that he was trash, and upon being reincarnated begins down the road of being a successful and decent person. The execution is much more questionable. Namely that he was a NEET with pedophilic tendencies whose relatives promptly evicted him after his parents died. He dies soon after when he pushes someone out of the way of a truck in an attempt to do something worthwhile with his life. Then gets reincarnated as a newborn in another world with his full memories of his previous life and mostly the same personality.
Some people are unable to get past that the author went as far as making him a pedo in his past life, or that he remains kind of pervy at the start of his new life. Even though those are apparently minor enough elements that the summaries you can casually find for the series don't bother mentioning it. Those same people also really hate that it seems to be more or less living up to its expectations.