I often see users, including moderators, leaving "helpful hints" comments on various posts. They could be posted on off-topic homework problems pointing the user in the right direction on how to solve the problem, but I also see them on ligitimate posts in the sense of "you should think about this to arrive at your answer"$^*$.
Now, I don't see where this type of commenting falls according to the help page about the commenting privilege. I don't think it falls under any of the three "When should I comment?" reasons, but I also don't think it falls under any of the "When shouldn't I comment?" reasons either. So where should this type of commenting end up?
As has been expressed before, answers in comments are not allowed because they cannot be down voted, thus preventing incorrect content from being explicitly labeled as such. It seems like "helpful hint" comments would also fall into this category, as no one can down vote incorrect hints as well. I feel like one should just make an answer if they have a helpful hint.
At the same time, though I'm still not convinced of it, others have argued that such comments fall under the "Leave constructive criticism that guides the author in improving the post" reason. IMO such comments only help the user arrive at the answer themselves and don't have anything to do with improving the post itself, but I suppose it's still something to consider.
I feel like what sets PSE apart from other sites is the lack of comment clutter, thus giving more attention and emphasis to good-quality questions and answers. Of course, maybe I'm just taking the comment policy way too seriously, but this is an edge case I've been wondering about for a while.
$^*$As an example, consider the question
Why do two objects in a vacuum in a uniform gravitational field experience the same acceleration even if they have different masses?
"Hint comments" might look something like the following examples.
Hint: Set up Newton's second law with the appropriate forces.
Have you tried applying Newton's second law to this scenario?
Think about what the gravitational force is proportional to.
Think about what Newton's second law says about forces and accelerations and how they are related.
Of course in reality I have seen more elaborate questions with more elaborate hint comments, but those are harder to contrive and still be more general, and I don't want to go looking for examples and call any user out in particular.