As far as I know, Ninja Slayer was never very popular. Before the anime aired, the novels were talked about as if they were more of a moderately funny joke than actually good works of fiction. Maybe the anime was doomed from the start, but in any case, I don’t know many people who watched it all the way to the end. The most common reasons for people not liking it appeared to be the heavy usage of flash animation and the fact that some just thought it was like an inferior copy of Inferno Cop. Both of which are, in my opinion, relatively legitimate concerns.
The reason that I think they’re legitimate concerns is because Ninja Slayer was adapted by Trigger, so the comparison to Inferno Cop is warranted (especially when it comes to the animation and some of the absurdity) and, considering Trigger can definitely produce good-looking shows with intricate animation, the “low effort” look of Ninja Slayer could feel like a slap in the face. But, in my opinion, I’m glad that it was Trigger who produced the adaptation and no one else. At its core, Ninja Slayer feels like it’s supposed to be a parody not of any single show or storytelling method, but of a greater amount of traditional story elements like ninjas, samurai, yakuza, etc, and in my opinion Trigger are probably the best at stuff like this.. The show also mostly references larger aspects of culture and history like capitalism, communism, folklore and internet culture rather than specific works of fiction. I found this very refreshing, and it was one of my favourite aspects of the show.
That said, the show I watched also had an aspect to it that was somewhat separate from the original version of the anime: The subtitles. Some translation purists would probably have an aneurysm if they saw the amount of jokes in the translation, and especially in the “TL notes”, which were full of meta humour, direct references, “worthless” information and sometimes straight up nonsense. Yet the TL notes, included in the version streamed by Viewster (great service by the way), were probably the most entertaining aspect of the show to me, and it was probably this that kept me watching even when the dialogue itself was less funny on occasion. Well, to be honest, nothing could really protect the show during its somewhat objectionable sex jokes, but most of the time the dialogue was pretty decent (though occasionally juvenile with vomit and pee jokes), and the narrator (who provided most of the funny dialogue) was consistently good.
In addition to all of the humour and the references and the TL notes, the show also had a few seriously moving scenes, especially during the Silver Karasu story arc. The themes surrounding personal loss, impending death and finding one’s place in the world were particularly decently presented, though these moments were definitely few and far in between compared to the humour and mindless action. But I think they show that the Ninja Slayer anime was not thought of as “just a joke” on the creative end. I’m sure it would have been fine had it been considered that, but I was happy to see that it could be a multi-faceted show even with its bare-bones animation and somewhat incoherent story.