i'm not a major fan of genre changes if they're large vs small. To me small is slice of life drifting to drama or reverse. Just an example but slapstick going to tragedy or tragedy switching to slapstick to me isn't going to work. To me the first volume was kind of slice of life along with growth. Then it became more of an action, fight story with some humor but not enough and definitely a different feel. You may have been bored with the slice of life but many weren't. To me a story where the guy falls in love with a succubus and she falls in love with him has so many slice of life directions it could have gone without getting boring that a genre change wasn't necessary. Maybe a better word then genre shift is direction or feel. To me major genre changes are hard to make work well. I don't think the author made it work well. I also got the impression that the author had a lot of control so this affects my future expectations of him.
In vol 1 there were three main characters, at the start of vol 2 one of the characters is dropped. After 14 chapters unless I missed it she is never even mentioned let alone the absence explained. To me that is another main issue with the series. To me it screams bad writing.
Even the author didn't seem to like vol 2. From the author in his notice (what a trifecta):
So in the end both the artist and I ended up making a series that I didn't enjoy writing (beyond some Lucifer scenes), and the artist didn't enjoy drawing, and the readers didn't enjoy either.
He also notes reader reaction was mostly negative. To me this was a failure. If you are going to do a genre shift you need to do it well enough that as least one from this list should enjoy it, author, artist, majority of the fans.