The whole premise of the story was just swept under the rug in the last 2 chapters. Honestly, I don't think anyone thinks one of them will actually eat his heart. Since the story and plot setup at the beginning doesn't necessarily remain the same when the story reaches the end, this ending was somewhat predictable in a way. It was definitely rushed don't get me wrong, the zombie hime just happened to have a medicine that reverses the Banrikou which also solves his heart problems, and the anti-climatic fight with Rena that just ends by cutting away.
I really would've liked to see a chapter developing Rena, we had chapters for Shouko, Lenna and even Hina but I don't think we got anything for Rena, the Black Onihime. The author kind of set her up as the one that Setsuna usually calls, so I expected her to be significant in a way. However, since there was no victor... nothing came of it. But then again, we never got an explanation as to why he was able to summon the 2 of them at once. A lot of wasted potential is what I'm getting at.
Though I definitely still enjoyed the rest of the series, thanks to all the translation groups that picked this series up, and I hope more people stumble upon this like I did.