With Dukkha we can combine it with Compassion/Lovingkindness but not all Dukkha reflection is actually Compassion/Lovingkindness reflection, in the sense that we do a formal Compassion/Lovingkindness wish. Compassion may arise but is not the main focus of the meditation.
Same with death. Let's say we think about death. How can we die here in this Wat? You can go for four hours if you can get your imagination going and you just think of one way after another, after another, after another. Then that is just death reflection, even though compassionate understanding may arise while we are doing this. We do not change the meditation by doing a formal wish, as we want to continue to deepen our insight into death.
How many different types of Dukkha can you have? Just go through your body. What can happen with your toe that is Dukkha? What can happen with your feet, legs, hips, etc.? When you do that for one whole hour and that is just a Dukkha reflection for the hour. Even though compassionate understanding may arise due to the understanding of the potential we and anyone with a body has for Dukkha, the focus is to continue to deepen this understanding of Dukkha rather than increase compassion and lovingkindness. The purpose is different.
So when we do Dukkha or death reflection alone the purpose may be different to when we combine it with a Compassion/Lovinkindness wish.