Absolutes

I'm not sure how "good" of a question this is, but it was something that I keep thinking about from the exercises. Chapter 2 has the explanation about words like "unique" and "critical" being modified incorrectly. I was wondering if the explanation means that all verbs falling under the nature of being "something or not" (ie: it is unique or not/it is critical or not) can never be modified with words like most/more/extremely, like in the example? Or is it a case by case basis, depending on the word (so words like "unique" just can't be used with "most" or "more" but words like "critical" can)? Can't you say one person is more critical than another? Is there some kind of rule for this?