How can double negatives "has a respectable history as a rhetorical device for emphasis" when they're harder to understand?

These two quotes contradict. I hate multiple negatives! Waste of effort, time, space, words! I have to stop reading and spend like 15 mins. reasoning to the positive meaning!

But how can double negatives "has a respectable history as a rhetorical device for emphasis" when they can be "incomprehensible combinations"?

Page 166.

F. Use double negatives sparingly for understatement. The double negative, such as “not infrequently,” has a respectable history as a rhetorical device for emphasis. Although it has fallen out of favor, it may still be employed on occasion as a means of understatement. For example, “not wholly unsuccessful” understates a lack of success, thus emphasizing it, as compared to its affirmative form “partially successful,” which emphasizes a degree of success.

Pg 201

C.WHEN POSSIBLE, MAKE SENTENCES AFFIRMATIVE, NOT NEGATIVE

We understand affirmative statements more quickly and easily than negative ones. In studies of the effects of language on the mind, researchers have found that affirmative statements are psychologically more linear than negative ones. With negative statements, we must first understand the affirmative sense, then negate it. This is analogous to understanding another language by first translating it into one’s own. Avoid these nearly incomprehensible combinations: “not otherwise,” “never unless,” “none unless,” “never otherwise,” and so forth.

Bahrych, Merino. Legal Writing and Analysis in a Nutshell 5th edition (2017).