Intellectual dishonesty charges (as opposed to simple charges of lying) require information about other people's internal states that I don't see how anyone other than the person can be privy to.
Let's take someone who believes, quite firmly, that abortion and infanticide are justified. These activities, according to Peter Singer, were attacked by people with Christian assumptions which need to be questioned. That is what the person says, that is what the person believes, that is what the person argues for. They offer criteria for personhood which fetuses and infants flunk, and they are bloody consistent about it. I may think they're cuckoo, but how do I get to intellectual dishonesty? What does the charge of intellectual dishonesty amount to here?
When John Beversluis's book C. S. Lewis and the Search for Rational Religion came out, a lot of people friendly to Lewis suspected some sort of dishonest effort on Beversluis's part. How could he say these things about Lewis? How could he fail to find in Lewis what we more sympathetic readers find? Beversluis's subsequent review of A. N. Wilson's biography, and his 2007 revision of his own book, make the charge of intellectual dishonesty very difficult to defend.
It's tempting to say to someone you disagree with, "surely you really know, deep down, that you're wrong about this." But how do you prove such claims? Intellectual dishonesty claims invariably poison dialogue, they make parties less willing to engage the discussion. A price in civilized discourse is paid when these charges are made. (Look at the quality of discussion in the Intelligent Design debate if you doubt me). That's why I think intellectual dishonesty charges carry with the a heavy burden of proof, and most of the time they are not worth making.