I think the reason is that potentially, there is a risk of getting an infection. The depleted-vitamins story is indeed nonsense.
If you get an infection, it will have to be treated with antibiotics and obviously, it is wise to avoid taking any medication when breastfeeding unless it is completely necessary.
With proper disinfection and a normally functioning immune system, the risk of infection is extremely small and none of our more than ten thousand customers has ever reported an infection. Countless people all over the world are dermarolling.
To minimize the risk, do not dermaroll extensive areas in one go. Do not roll all your stretch marks in one go. Wait for the rolled area to heal completely before you roll another part of the abdomen. And of course, thoroughly follow our disinfecting instructions.
If you are pregnant or breastfeeding, you should not use any numbing creams.