As someone who followed it from the first year it was publishing I can kinda summarize
So there were always issues from the beginning, but a lot of us were significantly younger and kinda just overlooked or didn't notice some of it at the time. Nakaba had made Kongou Banchou, which was a really fun straightforward action manga, so he had some good will going in. His art's really good, there weren't really any contemporary medieval fantasy shounen out at the time, and it starts off with a lot of mystery because the main characters are scattered and framed for a murder, there's this whole Demon angle that they spend a really long time building up to, and Nakaba for the first 80~ chapters was really meticulous and consistent about small details in the worldbuilding. It had some pretty nice and exciting fight choreography and the secondary mc Ban was just kinda universally seen as "cool". It had a lot going for it, but reading something at 17 vs the hindsight I have now and with a lot of Nakaba's more disturbing tendencies revealed, there were tons of red flags back then that I just kinda handwaved at the time as "whatever this stuff happens in manga" but spoiler: for some reason Nakaba's obsessed with making characters into ****philes at random. I genuinely have no clue why and of course I hope he isn't an irl creep, but it's so consistent in NnT and its sequel that it's practically all he's remembered for now.
On a strictly plot level, something weird starts to happen around 100 chapters in after the first major arc concludes. It's a gradual slide, but more focus is given to the Demons which should be a good thing, but the story starts to focus on the main character to such a degree that it actively ruins every other character. It's hard to communicate why this is so awful without actually reading it, but it's jarring because the first 100 chapters give basically every main character a fair shake, a range of fights, motivations, things to do etc. The second and third chunks completely remove almost all of this to focus on Meliodas, but the problem is he isn't even likable, his defining character trait is being a creepy pervert, but you were led to believe he had this tragic history and deep backstory that would tie into everything else. And he did. Sort of. Once we actually find out what it was, it's a combination of the worst cliches you've ever seen and the worst possible resolutions you can imagine. Those 10 Demon Lords we just introduced that are supposed to be like nothing in the world now? Meliodas was secretly the most powerful figure in the world all along and just was missing his powers, and effortlessly beats them at every turn. That dramatic backstory? The demon involved in the majority of the arcs and planning and suffering gets revealed to basically be a lesser Demon compared to the main ones, except he's the only one who has actually done anything plot relevant and thus the one people are actually invested in. He gets killed unceremoniously in a one chapter one-sided fight.
Things basically don't get better. The plot points continue to just suck ass, the reveals are increasingly disappointing except for this one weird Angel plotline where one character is revealed to have been a completely different person the entire time but even they weren't aware of it. Would have actually been interesting if the manga wasn't awful by this time. This is coupled with continually more creepy developments. Meliodas' infamous "get your hands off my woman!" line while holding an infant is the tip of the iceberg. In hindsight protagonists like King fell in love with other protagonist Diane when she was 9 years old. We get an "amazing reveal" that Merlin all along was in love with Meliodas because he's so super cool and powerful and also that her true form looks like a small child and also that she stole a bunch of power from the Demon King and Angel Goddess because she literally said "can I have some power" and just walks off with it.
It is genuinely the worst manga I've ever read and everything above doesn't do justice to how bad it actually is. The sequel doubled down on ****phile characters by having other characters even comment on it in series. I don't know wtf happened to him.
