Nobody fearmongering in this thread seems to have any idea how it works.
A brain chip would connect to a specific region of your brain. For example, in the case of a physical disability it would only need to connect to your motor neurons using read-only connectors.
If you specifically had them implant read/write connections into your visual cortex, then sure, someone could hack it and play ads in your head but that's kind of on you at that point.