I recently declined to do a tenure evaluation. The candidate involved had taken issue with me in his book, and I didn't want to do an evaluation that was mostly a debate surrounding particular issues of great importance to me. It didn't help that the person didn't cite my major article in which I take this position, using instead an older one. I also passed on an opportunity to write a book review of this same book. I could have pretty much demolished the book, but that would not have been very nice.
I recognized my own bias: I tend to think that scholars who agree with me are a little bit smarter than those who don't. Compensating for the bias, I would have had to write a glowing tenure letter saying how of course this scholar was correct in trying to refute me. I just didn't have it in me, though. I decided neither to help nor harm.
I also wrote a very scathing review of a book on Lorca in the last issue of Revista Hispánica Moderna. Here the ethical issue is improving the quality of the field by pointing out work that was not very good--in my opinion. The author of the book already has tenure, so I wasn't squashing anyone's career. I also had no bias, no stake in this particular debate.
I've done other scathing book reviews in the past. I've also been on the receiving end of a few negative reviews, and survived none the worse for wear. If I take a controversial position, I have to be expecting people to disagree with me.