Masterclasses are an interesting conundrum. I have participated in a few and watched many more. The problem is that it's just one lesson with no followup and there is only so much you can do in one lesson.
I have noticed that many pianists skip giving technical advice in masterclasses and I can sort of understand why. If there is a fundamental issue with technique I find it unlikely that it's something you can just fix in one lesson. And as I mentioned there is no followup so the pianist cannot check that the student has properly understood what they're supposed to do. I have made the mistake once of blindly trusting that I correctly understood technical advice I was given in a masterclass, and sadly messed up some aspects of my technique quite badly because of that (the advice was given by a very good pianist so it's me not understanding the advice rather than the advice itself that was the problem). In short, if somebody has big technical issues I don't think a masterclass is the proper forum for solving that. I remember another masterclass where we worked on chord technique and there simply wasn't enough time to look at it carefully and make sure I had fully grasped what I needed to do, we simply needed to move on to not have the audience sit and watch me trying to play a chord for an hour.
When it comes to musical issues it could work better, but at the same time I have noticed that when there are big musical issues with what a student is doing it often becomes clear that they're not really hearing what's wrong, and that it's yet again something you need to work on for many lessons, to develop hearing and understanding of how you even construct a phrase based on rhythm, harmony, upbeats and so on. Sometimes, musical issues are also an issue with technique, where you simply don't have the command over the instrument to easily produce the desired musical results. Then again, you can try to sort of chop away at it during a 45 min masterclass, but in reality, what's needed is to develop technique quite a bit further so the desired results start happening on their own. I can certainly relate to getting feedback on musical stuff I'm well aware of - like I could hear on the spot that the music didn't come out as I intended - but the issue was a lack of technical resources, not that I didn't know how it was supposed to sound.
With those things in mind, all that remains is to bring a student who is already quite skilled technically and musically to work on interpretation and understanding the particular piece using the experience of the master pianist, rather than working on fundamentals. The lesson tends to be much more productive in terms of quantity of information that gets worked through and relayed to the audience in those cases. And keep in mind, in many cases the purpose of a masterclass is often part lecture/entertainment, meaning that you want to keep things moving and give as much as possible to the audience.
To clarify, I'm not opposed to having masterclasses with students who have technical and musical issues. I'm just relating some of my experience of why these things can be problematic.