I have nettles growing as weeds on the perimeter of my property and it has never occurred to me to regard them as any kind of security. Even though they are fast growing. Their defects would include:
- they die down in the winter
- flimsy physical barrier
- sting cannot penetrate strong clothing
There are various types of hedge and shrubbery which are hardier in the winter, which can throw up a pretty impenetrable barrier, and whose thorns can do serious damage.
Blackthorn (which is seriously thorny and dense) and holly come to mind. This RHS page has a longer list.
Side note: my father, as a farmer, used to pull up nettles with his bare hands. It's not a heavily armed plant.