The thing I really disagree with is the qualm about how the friendship aspect plays out inconclusively. In the end, the manga isn't really about friendship but rather how the characters' relationships with others affect their growth and happiness. The reason I think the ending works, at least in regards to Chihiro and Yugami's relationship, is how it ties in to the title and themes of the story. My belief, at least, is that there's a good chance this is how Sakura planned it to be when all was said and done because the title is so deliberate.
In English the title's simply translated as Yugami Doesn't Have Any Friends but in Japanese, the grammar indicates a clear emphasis that this is Yugami's stance on the matter and leaves implied room for a contrasting view. It's why Chihiro's word-for-word title drop at the end of chapter 80 is so meaningful; Yugami may not think of her as a friend, but Chihiro considers him to be her friend and she's able to come to terms with that rather than focus on the negative that he doesn't reciprocate the feeling. Her ability to accept and be content with that also ties in to Yugami's valedictorian speech in a way: you don't need to waste your time fretting over your relationships with others. Just do you and be happy.
It's also worth noting that one of Chihiro's recurring thoughts throughout the story is her lament that Yugami will forget about her once they graduate. Considering he sent her that new year's card, thought of her when nobody believed Heiraku had created a story based on himself and wanted to continue seeing rakugo shows with her, it's clear that Chihiro isn't someone Yugami would forget about. Maybe it's a bit different from how he impacted her life, but she ultimately made her mark in his life all the same.
It wasn't so much about them being friends as it was about how their relationship, whatever they want to label it, helped them grow as people.