The problem (well, one of the problems) with Vinland Saga is that the protagonist is pretty bad: He starts out bad, becomes merely tolerable, and then ends bad. The show seems to be aware of this and as result shows him as infrequently as possible… which is of course another problem. The best character by far ends up being the protagonist’s sister, and she only appears in like 3 episodes. All the rest feel out of place in one way or another; Askeladd ripped from a Roman epic, Ragnar from an English play, Thorkell from One Piece, Canute from Berserk. None of them feel like they should coexist in the same creation.
Also based on a manga series, Kimetsu no Yaiba in contrast actually comes across as a self-contained work. The problems here mostly stem from the worldbuilding being poorly thought out, the plot developments being boilerplate battle shounen, and Nezuko’s identity consisting of little more than being brainwashed luggage. Pretty much the only part of it I really enjoyed was the (over)reaction humor since most of the action scenes are polluted by overly obnoxious graphic effects.
