It seems to me from scripts that handcuffs have 1% chance of breaking per turn for every point of victim's strength above handcuff strength (that is, 40/60). So, 50 base strength + (extra strength from difficulty), means ~10-15% per turn for assassin. Considering their cost of 40 TUs (+ need to free hands), seems like a waste of time.
As some wrote, it's a bit more relevant in BAI as waking up enemy units act immediately, but it seems just extra safety layer over babysitter that you'll need anyway.