It CAN stimulate tissue growth. it causes swelling/enlargement that can become permanent depending on the person. If you had inward turned nipples before they might become permanently turned outward and the shape will change, they will become bigger. For me, I liked the change although i will admit it is a bit lewd and I have to wear a bra if I want hide my nipples.
No way in hell does it kill sensitivity because putting a piercing in your nips does not remove or sever any nerves.
As for causing scar tissue... like why do you think that? If you go to a shitty piercer who uses unsanitary equipment and performs an unsafe piercing then yes bad things can happen... which is why you go to someone you trust.
The random fear mongering here is crazy. Nipple piercings look and feel great and I've had them for years. Just DYOR.