A few random thoughts, not directly related with this thread:
1- Choosing a keyboard is like choosing a car or a photographic camera or a cell phone or a wrist watch: it tells something about our own personality and our likes and dislikes (for example what kind of music we tend to play). Hence the negative reaction to criticism of any kind: ("How dare you criticize who I am??")
2- Criticism is more accepted when it comes from someone who owns the same keyboard (or the same make, at least), because in this case is also self-directed ("Look the mistake I made!"). Besides, sharing a common destiny establish a bond of some sort ("We are all in the same boat, aren't we?").
3- There are different ways to convey a negative judgement (see? I didn't even use the word "criticism" here!) A good idea is -like always- to sugarcoat it with an introductory phrase like: "It's a fantastic piece of gear; pity that...".
4- The least accepted kind of criticism is the one that looks or sounds like a mockery: "How could you buy that piece of sh@t? What a dumbass you are! Everybody knows that it has the "X" defect!" This last kind of remark can arouse homicide instincts (no kidding!) and -at the very least- end a friendship.