Not to say CAN'T WORK on a super-slow plane, but,,,,,
This year I noticed that F2D planes (100 MPH nominal) when involved in a serious line tangle and an airplane got slowed to "maybe 70 MPH", the shutoff shut off, thereby minimizing the harm of the tangle. So if the initial airspeed is 70-to-75 MPH and the manouvering speed is 50-60 MPH, you don't have much G force or line pull to work with to make an effective shutoff.
ps:
The Mears shutoff (same principle as the Viko) does not have an arming device, but requires the mechanic to roast his thumb whilst holding the airplane and shutoff, works OK 100% of the time: launch, combat, and cutaway.