In my experience, the difference between if and unless has been less of a problem than overly complex conditional expressions. As you point out, adding a well-named function or variable that summarizes a series of tests is more important.

Since we've probably all seen these same kinds of confusions in languages that do not have an unless, it seems more reasonable to focus on clear conditionals rather than which keyword is used. If the conditional expression is clear enough, the code will be clear.