These wizards would still have to beat someone into submission before they can control them
Do you think that it is hard to do?
Do you know that even today people are turned into slave, and to force them to be obediant they are beat up continuously until they dont even try to go against any order.
Men slave that work themself to death, literaly, do it because they were brainwashed this way.
Women who became sex slave and never do anything to go against their rapist were also brainwashed the same.
If she create this spell the only result will be inocent being turned into slave.