Some time ago I did a big research project, looking into the concept of music performance assessment, and different approaches to it. I read a huge number of studies and commentaries. and conducted some research of my own. The broad conclusion we came to was that following a strictly criteria-based assessment system - with no room for any subjective judgement - did succeed in bringing every assessor into line. Everyone gave the same performers the same results, and as such there was consistency and "fairness", of a sort. However, in a number of cases, they felt they were giving results that they did not actually agree with - as in, by following the criteria, it churned out a number - and each one, by following the system, came to the same number - but none of them agreed that that number was what the performance actually deserved!!
The conclusion of the project was that music performance simply cannot just be broken down to a sum of its parts. You can't create a system that is both 100% objective and "safe", and also accurate or "musical". Yes, allowing an examiner to make an expert judgement brings about a greater risk of mistakes or bias (and complaints!), but it is an infinitely more musical way to approach things. So any good system needs to hold these two things in tension. Yes, absolutely, put in safeguards, give training, use criteria / grade descriptors, aim for fairness and every examiner applying the same principles, and so on. But let the examiners do their job. If you have examiners marking too harshly, too kindly, or inconsistently, the answer is to better train or replace the examiners, not to give them a word bank and reduce their job to a box ticking exercise.
If an examiner is afraid to make a comment for fear of a complaint, something has gone seriously wrong with the system. Especially because, what harm can an encouraging comment ever do? Are they afraid of complaints if someone gets a low mark and yet the examiner says "Well done"? Better to say nothing in case their positive comment could be used against them? I agree with maggiemay - an empty box looks like something is missing. If the comment box is only or primarily used in "borderline" cases and fails, then it has essentially become a box for the examiner to justify their decision, rather than a place to offer some encouragement. Why are they being actively discouraged to give even a "Well done" - do the board not trust their own examiners to write appropriately? If an examiner is not trusted to write something suitable in that little box, why are they trusted to fill in the rest of the marksheet? I can totally see elemimele's point, that it's a response to current culture. But there has to be a better way.
I could talk about this for a lot longer, but I'd better not! Interestingly, at the point I wrote the paper, I praised ABRSM for having a more musical approach than some other assessors. It's sad to see that changing.