There are all kinds. I've only used the Leaf&Rusher one which has very short needles and is intended for getting actives into the underlying layer of skin, not scar remodeling. The longer needle ones look kinda dangerous to me...and painful!
In my experience, I wouldn't go any shorter than 1.5mm for something like stretch marks and scars. I have a 1.5 and use it on several scars, and also on my face without any problem.
