Mind control/corruption is kind of a crucial plot point in the game. 'Samples' + training are used to create sex toys for the baddies, plus some kind of semi-demonic presence in the MC that overrides the serum, incidentally creating a nice little harem for the MC's amusement (and horror.) Plus the other mysterious girl that might cause super fireworks if she and the MC get together. It may be the reason Cucky's ancestors were able to keep things in house, as it were.
It doesn't seem like immediate or guaranteed control either, as evidenced by the ongoing tennis match between the MC and Sara.