As has been noted within earlier feature requests, assignment comments are kind of a black hole of instructor feedback. Students aren't notified and instructors don't know if students have responded. In fact, competitors such as ELI have built an entire product to encourage iterative feedback between students and instructors. Since nothing is being done to address this, perhaps a relatively painless fix could be installed.
I am suggesting that a message icon be added to the speedgrader assignment page. You click on it, and can message the students easily and directly within canvas with your comments. Instructors who want to use submit the normal assignment comments are free to do so. #feedback