"Not accurate" may be useful feedback even if it's false. For example, perhaps several such responses in a row would prompt the author to link to supporting data, or reword their comment to be clearer. The real question though isn't whether it's always helpful, but whether on average it's more useful than just a bare downvote. While some of the responses will be "not accurate", others will be an extremely actionable "link broken" or "missing word".

I think the real issue of requiring comments on downvotes is that it would either disallow anonymous downvotes, or encourage anonymous comments. Perhaps de-anonymizing downvotes without requiring a comment might be a better first step: if you aren't brave enough to have your name associated with the downvote, then maybe you shouldn't be downvoting. I'm sure this would cause its own problems, but I suspect it would be a net positive. At times I think it might even be good to have all voting and flagging actions on HN to be public record.