For me, the biggest flaw of this story is none of the characters are likeable. Ayumi has the best background story but her negative way of thinking just gets in my nerves sometime. Jindou had been down right annoying until her back story was revealed and her character developed but I think it was to late to salvage her character at that point. Tenshi? She's just too obviously suspicious. I didn't like her character until the end.
And Kouki is the worst. He didn't do anything for the entire story. Everything was handed to him. I don't even know why he was even the main character.
Of course I'm agreed with the author for the story to end with Ayumi. She's been suffering for very long time. If the story ended with her heart broken, that would be very cruel. But only if he made Kouki and Ayumi more likable, this manga should have been more successful.