Thing is mafia can NK any time and stalker keeps receiving reports(if the setup does not include cop or any investigative role from town side.) This forces town to lynch to avoid stalker catching the PR or the important role they want alive.
So if it's a 5 way and then you lynch stalker will exploit NK and stalk til he finds the right pr.
Seen this exploited in Voltron Doesn't Like It where mafia NK's the very beginning and keeps stalking til they find the bulletproof. In that case they hit bulletproof so town loses a PR.
Can be exploited with setups that include bomb, oracle or any town PR like that where stalker can NK safe from hitting those and keep stalking so he knows what to avoid.
Can easily be exploited in Sweet Dreams and maybe other setups like lolnoobtraitormafialose or Jingle All The Way.
The point is, while I didn't say TOO many people exploiting the role like this, what if they do? This could break the balance of said setups and it's not hard to do it. It's beneficial so why not?
Can you fix this exploit by maybe adding a feature to the stalker that makes his ability useless if mafia decides to no kill that night?