That's true. But neither Juujika no Rokunin nor Pumpkin Night take themselves that seriously. The issue with the former is that it's purely a revenge series. Revenge can make for a good story, but is generally better to be part of a bigger story. HxH does the revenge stuff really well with Kurapika. His primary goals are retrieving the eyes of his brethren and killing the phantom troupe. But to get there, he often has to do other things. I think the same thing is true for romance. I like romance as a genre, but I think romance as part of a larger series is so much more enjoyable because of a bigger cast. In romance manga, you'd barely even see the friends of the protagonists, and conflict would usually come in the form of a love-triangle (which imo is an annoying trope and I just drop the series if it's employed). 2 people in love with one another also interact with other people, and those interactions can influence the relationship, tell more about it etc.
As for Pumpkin Night, I think the author just didn't really know what to do. I think he should've done a-western-slasher-but-as-manga.