The correct way to implement a STOP pushbutton is to : (i) hardwire it onto a Normally Closed contact and (ii) Program it as a Normally Open contact.
If it's implemented as you've shown ie. hardwired to a Normally Open contact and programmed as a (NOT) normally Closed, then it's unsafe because if a cable becomes detached from the Stop Pushbutton there will be no indication on the ladder logic.It will only become apparent when the stop pushbutton is pressed (possibly as an emergency) and nothing happens.
If it's implemented as you've shown ie. hardwired to a Normally Open contact and programmed as a (NOT) normally Closed, then it's unsafe because if a cable becomes detached from the Stop Pushbutton there will be no indication on the ladder logic.It will only become apparent when the stop pushbutton is pressed (possibly as an emergency) and nothing happens.