Yussel, the Gemara in Kedushin wondered if women are Mechuyav in Mezuza. Then, the Gemara says that it can’t be that they aren’t Mechuyav, since they also need a Shmira. This is besides the Gemara about Rebbi and Antoninus, and Unklos and the soldiers.
The Rambam had an issue with those who used to put strange characters into the Mezuza. This bothered the Rambam, and he said they are turning the Mezuza into some king of charm, which it isn’t. However, the practice seems to have been condoned by Rashi and the Yereim.