At our seminars, people often ask, or better yet assert, “If I kick him in the balls he’ll go down, right?” Well... that depends. Yes, attacking the groin is a valid target that we do advocate, if and when it presents itself in the context of Closest Weapon Closest Target. Meaning that if you are set up to easily make that strike happen with effective follow-through without telegraphing your intention, go for it!
If kicking him there requires an equivalent setup to an NFL pro kicking a field-goal, it is probably not the best target at that point in the fight.
The important point is that kicking a man in the groin should NEVER be considered an end-all-be-all. Unless you devastatingly destroy the person's anatomy with your kick (*cringe*), do not assume the fight is over.
If anyone tells you that “this strike” or “that strike” is a guaranteed knock out, or guaranteed to end the fight, etc., they might as well be trying to sell you penny stocks. Don’t walk away from their seminar, run. There are guarantees in violence and self-defense. You weather the ambush, find your indignation, and start destroying targets until your safety is assured. If you do end up striking the groin, don’t wait for the reaction, instead, immediately follow it up with more devastating strikes in rapid succession.