In the end, it's a very mediocre shonen with even less depth than most, but far prettier and better animated than most due to Ufotable's obvious influence - obviously amateur writing that fails to keep the initial sense of mystery and beauty intact. The main character initially seems like a departure from tired shonen tropes, but soon reveals himself as the most one-dimensional and unchanging Gary Stu of them all, pulling superpowers out of his ass with no explanation exactly when it's most convenient.
Nezuko starts off interesting - and then is completely forgotten about for most of the anime, offscreen for multiple episodes at a time, usually with minimal or no explanation, and then trotted back in when we need a cute scene or random ass-saving for Tanjiro. Probably the biggest instance of blatantly wasted potential in the anime and the incredibly poor writing sidelining her for long stretches of time so Tanjiro can be the star.
The two secondary characters introduced a fair chunk through are absolute irredeemable trash on a level I'm honestly surprised they even exist - the blonde one's only purpose is to be a gigantic pussy 24/7 while screaming loudly enough to give a headache (but it's ok, sometimes he hits on women as his only character depth) and the other bull-headed dude's only depth is running around trying to kick people's asses and yelling, which to be fair is less annoying than screaming, but only marginally. Neither of them develop in any meaningful way throughout the anime and come off as incredibly shallow and annoying caricatures, and none of the 'humor' involving them is the least bit funny.
In the end, KnY is just another mediocre shonen, with its main saving grace being the phenomenal presentation - it LOOKS gorgeous, and (no spoilers!) a few fights are an absolute joy to watch, but look past the gorgeous visuals and the show as a whole is a rotting corpse uglier than any of the titular demons that need slaying.
