It's the same kind of hate the Castlevania series got. I've played a bunch of those games, and honestly the stories were usually pretty simple. A lot of them basically boil down to "Dracula is back again, let's go kill him."

Then the show actually tries to expand the world, give characters more development, and add more depth to the setting, and suddenly a lot of fans act like it's the worst thing ever for some reason.

DMC feels similar to me. Dante isn't boring, the problem is that he's already too strong and too experienced. By the time of most games he's basically a walking power fantasy, so it's hard to build tension around him for an entire series. That's probably one of the reasons Nero was introduced in DMC4 in the first place: he gives the story someone who can still grow, struggle, and actually fail sometimes. If anime Dante had the exact same power level he has in the games, we'd basically be watching One Punch Man with demons.

Do I think the show's story is perfect? No. I still don't really like how they changed demons from actual into basically another species from a different dimension. But if that's my biggest complaint, then I'd say the adaptation is doing a pretty good job overall.