When the fuel is turned-off the engine tends to surge for a while and the aircraft oscillates in yaw, the stude is then forced to fault find by retarding throttles to determine which side to investigate further. Once they've discovered that a "dumb passenger" must have accidentally switched-off the fuel they simply turn it on again and reset the throttle. This is done in the cruise and I don't see that it's high risk, but if the FI wants the engine back in a hurry you just turn the fuel on - from the engine's perspective it's the same as when mixture is used. The risk is lower than (e.g.) a simulated engine fire leading to feathering (during asym 2) which isn't usually done over the airfield.
Maybe I've missed something, why do you think it's high risk?
Simulating an engine failure on the takeoff roll leads to huge sideloads on the nose gear and a VERY VERY small window for the stude to do the right thing - less than half a second! The FI needs to intervene so rapidly that the stude can't be left to make a mistake and to learn. If they ever have a real engine failure during the roll they will either react fast enough to stay on the runway or they won't - IMHO this exercise will not have helped them.
BTW, I'm talking light twins - many of which don't have Vmcg specified.
In the UK there's a test requirement for an aborted take-off but (
AFAIK) this is usually initiated by the FE calling stop, stop, stop. In my early days of examining I used mixture once, but never again!
HFD