Especially for the many of us using OER online textbooks, we are often posting direct links to relevant textbook pages or exercises in assignment feedback comments. SpeedGrader (DocViewer, technically) annotations currently fail to recognize links and make them clickable. Students understandably do not take the trouble to manually select, copy, open new browser window / tab, and paste the link, and thus fail to complete the learning cycle of applying instructor feedback.
This of course also applies to all Canvas instructors who simply want to make great use of their own Canvas material by pointing learners to specific Canvas pages as feedback to specific points in learners' submitted work.
The key learning/teaching distinction between DocViewer annotations and generic Assignment comments is that annotations focus the learner's attention on a very specific part of their submitted work. Giving that kind of very specific feedback is crucial for providing effective feedback in all subject areas. For example, in my field of language instruction, when a learner reads a specific verb in the wrong tense, I need to be able to send the student directly to the specific online textbook (or Canvas) page that addresses exactly the skill that they need to review related to that specific error.