Let's say a Rebbe is really mean but it WORKS! The talmidim are terrified of him and it is not out of love but out of fear that they learn. Of course there must be some element of fear in the relationship [just like a father-child relationship] but the fear should be yiras kavod, awe and reverence, and not fear of being slapped [physically or otherwise]. However the dominant element should be love. In an instance where the students are driven to learn by pure fear THAT IS NOT THE TORAH G-D GAVE US!
It works? In Aushwitz people worked so hard that after the war they displayed a tremendous work ethic that enabled them become rich. Did Aushwitz "work"? להבדיל. Does the end justify the means???
May the Jewish people always be blessed with loving Rabbeim.