I only ask as my ratings dropped like a brick
I, in 95%, of rejection of ratings. Always add a comment. That explains why I made a decision. My view is my comments validate the opinion - had they been read.
So do they actually make a difference or am I just wasting my time?
I used to leave comments when I first started reviewing…then reality kicked in and I got well and truly told that nobody read 'em. Just imagine the staff reading nearly 1,000,000,000 comments a year… how would we get bugs… I mean features in the games!!
This is what I do.
It makes me stop and think as I write am I being fair?
They can read them I guess if there is a problem……
So the chances they are ever read is low.
Under GDPR I asked for my data and it included these comments. I never check how it’s written so it was funny to try and work out what it meant 
I’m sure some poor analyst had to look at it once.
That’s probably about it
Certainly the submitter never sees them
So, first off, you can only leave comments if you reject for Accuracy, and it’s possible that may not be the right way to go about rejecting a nomination. Accuracy should only be rejected if the nomination has a bad photo/title/description, abuse, or inaccurate location. A comment related to one of the Accuracy rejection reasons can be given if needed.
However, say you get a playground located at a K-12 school; this would need to be rejected under Appropriate > Schools. Say you get a nomination of a statue in the middle of a roundabout without pedestrian access; this should be rejected under Safe. Say you get a memorial bench, and the submitter didn’t provide much context as why the person was important to the community; these I typically reject under Permanent and Distinct.
Edit reviews do still have the comment box, and those do get read by Niantic staff before a final decision is made. Phioto reviews do not have a comment box, and there are only 4 rejection reasons.
Since the new review flow was launched last year, many have asked for the comment box to be returned and included for all rejection reasons, not just Accuracy. Therefore, at this point, use your best judgment as to what is the best rejection reason, and try not to reject for Accuracy if that isn’t the case just to leave a comment; that could go against your rating compared to how other reviewers rated the same nominations.
It also is extra work for staff, and I think getting rid of the general comment box helps cut down on this work and allows the staff to focus on other tasks.