It's not just that giving a voteworthy supportive response requires domain knowledge — you also need something to add to the original article. I've seen lots of good articles on topics I know about, but usually I'll just upvote rather than comment because the original article did a fine job by itself. If I do comment, a lot of the time it will just be a minor clarification or tangential thought rather than something that merits a lot of upvotes.

Skepticism, on the other hand, is automatically something meaningful you can add.