Negative Reviews

I don't like to write them. The rarer action is to praise rather than blame, though, Prospero says in "The Tempest."
But for me the reverse is true. Mostly I write reviews of books I enjoyed, sometimes even loved with a burning passion.
When I do write a negative review, as in say this piece that came out today in The San Francisco Chronicle--
http://www.sfgate.com/books/article/T...
--I have a certain logic in mind. I say in the piece how much of an admirer of Johnson's I am, and that this book, in which he stumbles badly, is an exception. Which is precisely why, when asked, I wrote the piece. Over along period of work, half a lifetime's work, writers may stumble, and readers ought to know that before they blindly pick up a new book whether or not it is worth the time.
Or so stands the belief of a professional journalist, a reviewer for many decades, author of thousands of review pieces. Not many of those, a tiny percentage, come out negative. Perhaps sometimes because of a flaw in my vision, but mostly because writers sometimes stumble.
I know this is true because I am a fiction writer and journalist first, and a reviewer second.
I hope I haven't stumbled about on this subject.
Tell me, please, if you think I have.