And definitely, there are master classes and master classes. I've been at master classes conducted by famous performers who basically provided no useful feedback whatsoever, just listened to the student and then complimented them.
It seems to me if you want to lead a serious master class you need to be a superstar teacher, the kind who students fight for the chance to study with.
This is a good point. There are some performers whom probably haven't the first idea on how to actually "teach" a student, yet the world looks at the performer for playing tips that are magically supposed to be given in one session and change the student (for the better) forever ! hee hee ... I think that's comical.
It's obviously very possible for one session to make a world of difference to the student (and perhaps to the auditors and otherwise onlookers), but it will be always dependent on how good the master is at actually
teaching, and how receptive the student participants are to the teaching. Some performers have actually no idea how they do what they do, yet they must pretend as though they do or else by other people they would not be considered a master. Imagine a particular education venue hiring a fancy performer to give a concert, but the ("stuffy") performer is unwilling to share his/her expertise and work with the students there ? Whether performer knows how to work with students or not, performer will not decline giving a masterclass because it can appear to be a personality fault when in fact it may not be exactly as it appears. What an expectation !
Some of the more productive masterclasses that I have been to are ones in which a the class is structured around a specific subject
within the field, and the particular master is truly a master teacher in that specific subject. Then again, the original topic here really seemed to have nothing to do with whether or not the class is actually
productive 
.