I think a lot of people are being overly harsh on this article, which I think is a completely fair and competently written piece of criticism.

The central thesis of the criticism is that demons are portrayed with less moral complexity than could have been present so that it can be used as a plot device to better highlight the changing and more relatable main character. I don't disagree with this, but I also don't disagree with many of the comments in this thread about how this serves the story very well and that creating moral complexity in this case is unnecessary. Both of these can be true at the same time.

Creative works always exist within the context of their creation. Without the fantasy works that came before it, Frieren would have to do tons of dry world building to establish the setting because the readers wouldn't have any preconceived idea of what an elf, dwarf, demon, etc. is. In Frieren's case, there is the added context of the Japanese media landscape in which poorly done plotlines sharing similar veneers of morally gray demons are all too common. This makes Frieren, an excellent story which smartly uses a straight evil portrayal of demons to feel like a subversion (Himmel being unambiguously good is a similar situation). However, just because Frieren is subverting badly written versions of morally gray demons, doesn't change the fact that the demons in Frieren lack the moral complexity in the way the author is discussing.

In any case, this was an interesting read and helped me stay awake on the train this morning. Thanks for sharing!