Let me start by saying that I have NOT read every single word of this long thread. I’ve skimmed it and read bits and pieces, and I think I have the jist of what everyone is saying and feeling.
I’m kind of mystified by the original question, because I never had any teacher who drove me to tears and, while I am a very demanding teacher, I have never had a student near or in tears. I think that a teacher who drives a student or numerous students to tears has a personality problem, not a teaching one.
Having grown up with a parent who was a brutal emotional batterer, I am hyper sensitive to misplaced criticism. That is, criticism that attacks who the student is rather than what he/she has done. My particular area of interest in teaching harp is taking students who have come to me from other teachers(please, I didn’t steal them!) and fixing technical problems as well as moving them into more and more difficult repertoire. To do that I have to be very demanding and persistant, but at no time am I ever demeaning, sarcastic, destructive, or vindictive. In the first place, that would accomplish nothing. In the second place, it will create more problems than I am supposedly trying to fix, by making the student do worse rather than better.
A teacher who is mean, angry, demeaning, sarcastic, extreemly negative, is that way because of his own personality and not because he has to be that way to get the best results. A student who studies with such a teacher, even for a short time may take years to recover from the damage a person like that does.