Involuntary implants for behavior control of violent criminals, or just hackers taking control of regular implants, or worse still ... advertisers
It is not hard to come up with plenty of dystopian scenarios involving this sort of technology.
My thoughts are that each brain has their own neural network that is unique to the person. This would mean that each neuron, despite similar/same location, will respond uniquely, as our personal neural networks were trained separately (across many years/decades). While I believe it may be possible to train a system to output stimulation to invoke a response, it would be susceptible to the same issues as simple password hashing (that is, while you could break a single hashed password, it would be computationally inefficient). Ignoring quantum computing theory, I think this would then make developing a system that can accurately control someone in LONG sequences would be difficult.