In other words, they don't train the target muscles optimally in the sense that regular curls are optimal for the biceps due to their palms-up positioning.
While dumbbell curls aren't as good as hammer curls for building the brachioradialis, you can work that muscle with other, more specific exercises such as reverse curls.
However, this fact in itself is a benefit because, as a result of the neutral hand position, the movement is highly effective for developing thicker forearms.