I have just caught up on this conversation, and I have to agree that if it's what your writing in a recommendation to someone else who is looking to read it, then it's OK to mention both what you liked and what you didn't like.
As for comments, I never thought there was an "unwritten rule" about not giving constructive criticism. It's OK to say something like "I really like your story, especially the part where character X does thing Y. I thought the dialogue seemed a bit stiff at times, but overall I liked it." Something like that is still overall positive, I think, and I don't think a writer would be hurt by it. Just commenting "I hated this, you suck," is just mean and not useful at all, but that's not what you're thinking of doing, so I don't think you should worry about talking about what you think is a story's weakness if you still liked it overall.