Subcutaneous edema (which means right under the skin surface) doesn’t help with permanent enlargement.
The aim of pumping is to expand the tunica (and to some degree the CS and glans) for permanent enlargement. These are the erectile tissues and tunica expansion, over time, becomes permanent gain.
Subcutaneous edema has a few negative effects. Firstly, it is not useful for permanent enlargement since the lymphatics of the skin will resolve the edema fairly quickly and the skin goes from being thick back to its normal caliber. Secondly, the edema is unsightly, lumpy and squishy. Lastly, the edema actually causes some degree of temporary cutaneous nerve desensitization, and the sensitivity of these nerves is the major reason that sexual stimulation feels nice. So you kind of numb your penis a bit until it resolves. This is why most guys have some degree of erectile dysfunction while the edema is there.
Some degree of edema is probably unavoidable if you are using enough vacuum pressure to create tunica expansion enough to lead to growth over time. But it is sensible to limit this as much as possible and once you notice your penis is getting edematous the session is likely over in terms of useful expansion, since once the vacuum pressure starts to be more in the skin, the deeper structures, which are harder to expand, are no longer the main target of the pressure.