The comment that would be LEAST useful as feedback on a flash fiction story is:
"The story is so short that I didn’t get a chance to know all the characters."
This feedback is less useful because it does not directly address the qualities or effectiveness of the story itself, but rather the constraints of the flash fiction format. Flash fiction is inherently brief, and while character development can be part of the critique, the comment suggests a misunderstanding of the genre's purpose and limitations, making it less constructive.