So, I'm far to the left of most people, and I'm no fan of Trump, but I watched that video and I got the sense I usually get from reading about ML/big data, people seem to be selling these as more than they actually are. It almost is intuitive if not, I guess, what most people who study market research already know: you tailor your message to your audience. Even on a personal level, anyone who has navigated real life with real people knows that you have to communicate with people in the way that they will be most receptive. Nowhere are they "planting" ideas in people's heads, nowhere are they brainwashing people, they are merely saying the same message (for example, defend the 2nd ammendment) to people who might already agree in a way that they'll receive it. The big 5 personality traits are an already well known and documented idea, and even without that, in your day-to-day life, you learn what people's personalities are and you tailor how you communicate with them anyway.
The thing that they seem to bring to the table however is the massive amount of personalized data. That is the issue here, because that data can be abused by nefarious actors if it were to fall in the wrong hands. What usually takes a much more personal touch (like going out to voters, talking to them, or if you can't do that, have teams on the ground that talk to voters and tell you before your speech what they care about and why) can be done in mass due to essentially aggregating private data that users "agree to" obliviously or even don't consent to. That is the issue at hand, not like Cambridge somehow warped the people in Wisconsin's brains to mush and made them go to the polls.