Tbh those people were down on it because they didn't want it to be a SoL comedy. I guess given the first episode thats fair, but once Anya got enrolled into the school it became obvious what the show is. SxF does deliver on all the serious arcs when it gets to them. Dog bomb arc, Tennis arc and now boat arc.
It comes from manga readers and the promo making the show seem like something it wasn't. I'd know because that's the reason I checked it out myself, I thought the show would be something like a spiritual successor to Lupin the 3rd and it's not at all close to what Lupin is. Plus I think it was a main rotational manga on Shonen Jump which has it's own kind of prestige and standards that people associate with that.Yeah, I never understood those complaints because it was clear that the show was going to be a SoL comedy. It just sucks that crowd got their voice amplified, but I feel that happens with most animes these days.
Now that the show has been properly identified and catalogued, there's people who are still keeping up with it that are just complaining because they already invested time in it and want to see it through rather than dropping it.