Grading on the Canvas teacher app on my iPad is much easier than using SpeedGrader on my laptop.  But, if I want to leave specific feedback for the rubric components, you cannot...you can only leave a general 'comment'.

On SpeedGrader/laptop, there is a comment "bubble" for each rubric criteria.  As an art teacher, this feedback/comment for specific rubric components is more valuable than an overall comment.