I only ever saw the guillotine used in anger once.
I was on tow out duty and sitting in the cab of the winch with the driver during the launch.
The K21 launched and the pilot somehow managed to not release the cable, I have no idea how he managed it, but he started a turn with the cable still attached and as he had not overflown the winch the cable had not back released.
The glider actually pulled the winch sideways a few feet before the driver used the guillotine. Just as the he cut the cable it also released from the glider and it landed safely, but it was pretty interesting for a couple of seconds!
I dread to think what the loadings were through the hook, but I remember that the weak link didn't break.