For whatever it's worth from a non-Halachic mind...
I've long felt that using sechel is important. Obviously what the Chazon Ish says about it needing to be accurate is of utmost importance (as it always should be), but once that's established, I see no issue in reporting the truth and informing people about Rabbonim/Gedolim/et al. By definition a gadol needs to be someone whom a person can respect, and if knowing a piece of information will change a person's mindset as to how they view a Rav/Gadol and what they are saying I would think that is critical.
A few years ago, I was criticized by some for allowing to be published on my blog a [public] letter of a Rebbe where he spoke out in a very nasty way about a certain institution. Nobody disputes the accuracy of what was written, yet I received threats from some of his talmidim for 'saying badly' about a 'Gadol b'Torah'. I believe all of those people were wrong - there are many parents who would be hesitant at best to send their child to a Rebbe with those views, feeling that he will not teach their children appropriately, and it is important to show those parents what he thinks in his own words. I don't think this constitutes lashon hara whatsoever.*
Perhaps more importantly, a friend once went around to a number of Rabbonim/Rabbeim and Gedolim asking them to explain the concept of Da'as Torah clearly. He felt the best answer he received was from a Rebbe of ours who said that while he can't define the term, he can advise that a person find a Rebbe who "cares most about what's best for him [the Talmid] than anyone/anything else [i.e. the Rebbe, the Yeshiva, perceptions, etc.]". Based on that, which I think rings very much as Emes, I would further state that it's important that people understand which Rabbeim and Rabbonim and Gedolim are most capable of doing so. Therefore, I think it not just permissible but perhaps important to be clear about what a person's strengths and weaknesses might be particularly as it relates to being a Rav/Rebbe/Gadol.
I should add that it has often struck me that many Rabbeim I've met or even had are not capable from a Middos perspective of doing so, and that has always bothered me (perhaps particularly coming from a Mussar perspective, having grown up down the block from R' Dessler in Cleveland and learning in a Chofetz Chaim branch in a small town) - how can these people lead others, serve as their Rebbeim, etc.? (To varying degrees, and some people with poor middos can still accomplish a great deal in other areas of chinuch or Rabbonus etc., I don't mean to paint a broad brush at all.)
*A Rosh Yeshiva I respect who was directly negatively impacted by the incident expressed very nicely to me a couple of years later that my posting about the letter had hurt, and he felt I should not have done so, though he said he was fully mochel regardless. I disagree (and didn't ask for said mechila, though it's nice to have in case I'm wrong) for the reasons stated above.