Everything has flaws in it and this show has so much flaws!
Starting with good thing about this anime is that it has a great animation of its action scene which no one can deny that they are amazing.
They have spent like 1k$ on each episode but a penny on the story.
Those who have watched big 3, they all know that a shonen anime generally start with a character who had to go through a bad past, has a dream which he will accomplish in the end, lots of flashback, a rage moment where he will unlock new powers and main character being too good without any reason.
I don't say keeping these things are bad but I was expecting some new and real stuff.
But even these things are not put in the story in a better way. For e.g
Flashback- Every time a demon is killed they just show their flashback that the demon was not evil but he lost his consciousness as he became a demon, I mean the same kind of backstory everytime, as a viewer I got it with the first time you introduced the concept that Demons are no more human as they lost consciousness and so they are not evil, but they keep repeating the same kind of story for emotional attention.
Power Structure - Every Swordsman has different techniques and they are like very strong but they are still humans, how can they produce cosmic energy if they are just human, either you introduce such a power structure which enable character to be very strong or you don't bring cosmic power in the character and another thing about power structure is that it is too vast, demon has demon art and what kind of Demon art exist is undefined and there are many techniques used by Demon Slayer which is too large in numbers.
Rage moment- in every anime there is a very typical scene that the main character will unlock a higher power on getting exposed to a bad situation, sometimes it makes sense and in Demon Slayer, rage moment also makes sense but in 1 arch the main character unlock different Powers 4 time is just a lazy writing.
In the end, I will say please don't use brain while watching it otherwise you won't experience the great animation this show will provide.