The number one reason I heavily dislike this ship (or any non canon ship in this series) is that most people who actively ship them do so by stomping all over the established characters, their growth and their emotions, just to see their fantasies fullfilled.
This one wasn't too bad, but it still completely sidelined Touko, took a huge shit on Yuu and her feelings and denied Sayaka the happiness and closure she achieved in the novels.
It's by no means a bad piece by itself, some emotional bits were quite poignant and the interaction between these two felt nice and natural, but it did so by disregarding almost everything the characters stand for and achieved while dropping them into a situation where everyone came out worse than before.
As an original story with original characters, I'd even be quite the fan. But as something that's supposed to be a "fan" work of Bloom Into You, I really don't like it.