The wording is very vague, it does not give any specific powers, it limits any group formed to times of say full declared war or imminent threat to the Federation. Which is a very shakey legal ground to create a group that is from what we see in Into Darkness, operating out of every level of Starfleet and Federation beaurocracy on a non-stop basis, using technology ahead of the current fleet.
In unmarked vessels giving no IFF, able to commit acts of violence against the rest of the fleet and Federation representative officials, officers and vessels.
It's not just vigilantism, but taken to a staggering degree that verges well into facism, lawlessness, war mongering and the expressing of bigoted, violent elements of the organisations personnel and even becoming a form of terrorist group with members the likes of Terra Prime being radicalised and finding a safe haven within it to express their hate and anger.
Able to unilaterally commit acts of war against sovereign states that could lead to war far more than stop it, and persue everything up to and including the genocide of entire socities without any moderating of their actions from any outside source.