The use of a Dermaroller, which causes wounds, has its risks. It can introduce an infection because the scalp contains the potentially dangerous “staph” bacteria in a significant number. So, by making wounds with this instrument, it produces an unnecessary risk which can, on rare occasions, produce cellulitis. Cellulitis can become necrotizing, a very rare form of cellulitis that can become a life-threatening event.
There is no evidence that the use of a Dermaroller will provide any benefit to cause hair growth. I would caution all of the readers here to think more than twice about using this modality of treatment. There are some good treatments out there, see a doctor and get one that works.