As a writer with three traditionally published novels and a few other self-published novels, I'd say I would always prefer an honest review, even if it is negative, because I find "negative" reviews for my work are mostly positive reviews in disguise.

Example: This two-star review of my most popular novel to date, Children of the Folded Valley, from Amazon:

"Saying I enjoyed this book would be wrong. I found it disturbing, distasteful and fascinating all at the same time. This is the first book I have read about cults such as this and although it is a work of fiction the content is all too real. Read it if you will but to me there is enough sickness in the world without reading fictional tales of it."

Here's another "negative" review for the same novel, though this reviewer gave me four stars:

"I generally review only the books that I really love or hate passionately. I neither hate nor love the book, it has its flaws, but the story has stuck with me. This is a good, original story. The concept and characters are engaging and appealing. There are places where the writing gets a little thin, but I hope to see many more books by this author. It has a happy ending, of sorts, and yet left me profoundly depressed. I think it was that the author hit it right on the head; even in a perfect paradise, people are going to be perfect s***s to each other, given half a chance."

My point? My work isn't going to appeal to everyone, and I'm happy with that. Also the point made in the second of the above reviews, about the writing being thin in places, is fair criticism, in my opinion. Looking at it now, Children of the Folded Valley is a good reflection of where my writing talent was at then (it was released in 2014), but if I were to write the book now, there is at least one info-dump chapter that I'd approach differently, and I believe my writing is much better in my more recent novels.

I try to adopt similar honesty when reviewing other works, though if my opinion is too negative, I tend not to leave a review at all. I might contact the author privately, but only if my honest opinion is asked for.