The way I recall the exchange that I read, the case was the other way around: someone brought up feedback prefaced with "I usually hate pairing/trope/whatever", and in reply someone said that that was the equivalent of "I like this even though I don't usually read pairing/trope/whatever" - but these are two different things. "I usually DISLIKE X" does not carry the simple and neutral connotation of "I don't usually READ X", but the loaded connotation of "I dislike most of X". If either one of them is followed up with "...but this changed my mind about X," then it instead connotes "I used to consider X's foundation to be inherently wrong, but I now see the value in it." But on the other hand, stressing making an exception only reinforces that they don't like X.

This is in a sense a matter of "I liked your story a lot and here are my thoughts on yaoi", except that in this case X/yaoi is generally an object of the author's fannish love, which is something that many people feel emotionally involved with, and unsurprisingly, "I usually think the object of your fannish love is boring, or sucky, or distasteful, or irrational because it's not supported by canon!" doesn't go over well with them, even when it is brought up for the sake of making them a compliment.

Obviously these people realise that the intent was to pay a compliment, but that doesn't really matter if the other content of the comment pisses them off. The net result is not the gratification of having received a compliment, but the bruised feelings of having one's taste insulted (as the Fraser/Vecchio shippers felt in this particular kerfluffle). Since that is the opposite effect from what most people intend when they leave feedback, and it's kind of hard to know in advance whether someone belongs to the large group who identify with the objects of their fannish love so as to feel insulted by being informed that other people dislike them... well, that sort of feedback is a little unpredictable.